ion milling damage in inp and gaas s. j. pearton, u. k....

Ion milling damage in InP and GaAs s. J. Pearton, u. K. Chakrabarti, and A P. Perley AT&T Bell Laboratories, Murray Hill, New Jersey 17974

K.S. Jones University of Florida, Gainesville, Florida 32611

(Received 5 March 1990; accepted for publication 29 May 1990)

Near-surface damage created by Ar + ion milling in InP and GaAs was characterized by capacitance-voltage, current-voltage, photoluminescence, ion channeling, and transmission electron microscopy. We find no evidence of amorphous layer formation in either material even for Ar + ion energies of 800 eV. Low ion energies (200 eV) create thin « 100 A) damaged regions which can be removed by annealing at 500 ·C. Higher ion energies (> 500 eV) create more thermally stable damaged layers which actually show higher backscattering yields after 500°C annealing. Heating to 800 °C is required to restore the near-surface crystal1inity, although a layer of extended defects forms in GaAs after such a treatment. No dislocations are observed in InP after this type of annealing. The electrical characteristics of both InP and GaAs after ion milling at ;;;,500 eV cannot be restored by annealing, and it is necessary to remove the damaged surface by wet chemical etching. For the same Ar f ion energies the damaged layers are deeper for InP than for GaAs after 500 e V ion mi11ing at 45° incidence angle. Removal of -485 and -650 A from GaAs and InP, respectively, restores the initial current-voltage characteristics of simple Schottky diodes.

INTRODUCTION A considerable body of information exists on the char-

acteristics of ion beam etching of GaAs, due mainly to the extensive use of this technique for mesa isolation of GaAs-based electronic and photonic devices. I-lO In short sum-mary, it is usually observed that the etch rate goes through a maximum at an incidence angle of the ion beam of about 60°, with the sputtering yield being proportional to the ion ener-gy.I1.l2 These energetic ions create deep level traps which lead to significant carrier compensation at depths up to - 2000 A from the surface. At shallower depths the appar-ent net electron concentration in n-type GaAs can actually increase when ion energies above 400 e V are used. 7 In these samples, low-temperature (400°C) annealing can actually lead to further degradation of the electrical quality of the GaAs. 7 In general such low-temperature annealing leads to improvements in diode ideality factors (n), but the Schottky barrier height (<p 8) remains below its unetched value. 1>, I, Pang et al. showed that the ion damage depth is shallower with low energy and heavy ion species, as expected, and that the introduction of an adsorbed gas on the sample surface can act as a protective layer to further reduce ion damage effects. III

By sharp contrast to the situation for GaAs, very little is known about ion damage to InP surfaces. There has been one report aftne introduction of deep level centers in In!> during ion beam etching with 0.5-2 keY Ar" or C12+ ions.]4 These levels were found to anneal out approximately 300°C, and to be more prevalent in the case of Ar ion beam etching than with CI l etching under the same conditions. Electrical and structural changes in the near-surface (- 1000 A) of InP reactively ion etched in C2 H6/Hz or CCl2 F2 102 discharges have also been reportedY However, there have not to the

best of our knowledge, been any systematic studies of the ion milling characteristics of InP. It is generally true that there is a lack of such data for most of the technologically important III-V compound semiconductors.9

In this paper we report a comparison of the Ar f ion milling characteristics of lnP with those of GaAs. In partic-ular we have measured the average etch scale of both materi-als as a function of Ar f ion energy, and beam incidence angle. The resulting surface morphology was examined by scanning electron microscopy (SEM), and damage intro-duction was introduced by photoluminescence (PL), ca-pacitance-voltage (C-V), current-voltage (I-V), ion chan-neling, and cross-sectional transmISSIOn electron microscopy (TEM). The removal of this disorder by both elevated temperature (up to 800 °C) annealing and wet chemical etching was also studied for a variety of incident ion energies.

EXPERIMENT We used a variety of different types of material for these

experiments. Most of the etch rate, SEM, PL, RBS, and TEM measurements were performed on semi-insulating InP (Fe-doped) or GaAs (undoped) substrates that were che-mically etched prior to the ion milling experiments to re-move any remnant polishing damage that might affect the results. The electrical measurements were performed on n-type InP UVD -NA =6XIO]5 cm- 3 ) or GaAs (/VI) - /VA = 1.1 X 1017 cm - 3) which had alloyed rear AuGeNi ohmic contacts. After the ion milling treatments, front Au (for InP) or TiPtAu (for GaAs) Schottky contacts were evaporated onto the front face of the samples,

The ion milling was performed in a Technics Micro Ion Mill [Model MIM (TLA 20)] using an Ar+ ion beam

2760 J. Appl. Phys, 68 (6), 15 September 1 S90 0021-8979/90/182760-09$03.00 © 1990 American Institute of Physics 2760

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(200-800 eV) whose angle of incidence was varied between 0-750 from the vertical. The samples were mounted on a water-cooled piate held at 10 °C, and the chamber evacuated to - 3 X 10 - /} Torr using mechanical and cryogenic pumps. Typical neutralized Ar + ion current densities were 0.5--0.8 mA em -- 2 at the sample position.

The milling wafers were measured on samples patterned with AZ 1350J photoresist by removing the resist with ace-tone after the ion mill and obtaining the step height with a Dektak Stylus profilometer. The same samples were used for SEM examination of the surface morphology. Photolumi-nescence measurements were performed at room tempera-ture using Ar' ion laser excitation on un patterned samples. Capacitance-voltage ( C- V) measurements were made with a Hg probe contact and a 10 kHz Hewlett-Packard 4275A LCR meter. This method was reasonably successful with ion milled GaAs, but after milling of InP we could not make a rectifying contact using Hg, or with evaporated Au. Da-maged layers were removed by wet chemical etching in IH2 S04 :!H2 0 2 : 10Hz 0 for GaAs or IHCI:5H2 0 for InP. The current-voltage (1- V) measurements were all per-formed using samples with evaporated front Schottky con-tacts and the alloyed rear contacts. For ion channeling, the samples were run with a He" ion beam energy of 2.275 MeV, a normal detector placed at a backscattering angle of 160· and a grazing exit angle detector which intercepted ions scattered through an angle of 105°. In order to channel the samples, the backscattering signal from the normal detector (160°) was monitored while the sample orientation with re-spect to the analyzing ion beam was varied. When the back-scattering level was minimized the sample was "channeled".

1000 0

Ar+, 45°, ,aoe I I

900 / GoAs I

o InP / /

N 800 I E I u / ..... <{ / E 700 / .... I'

'" / -" 'E ,r! -- ... ..... 600 @« ,,-, ..-Y .... ........ w !;;i 500 e-/ 0:: ........ ,9 ::r: ...... , u 400 / l- e! W

W CJ 300 <( tr W

200

100

01 I I I I I 100 200 300 400 500 600 700 800

Ar+ ION ENERGY (eV)

FIG. 1. Average etch rate normalized to ion current for Ar ion milling of InP and GaAs, as a funciion of ion energy. The beam incidence angle was 45". and the sample temperature 10 'Co

2761 J. Appl. Phys., Vol. 68, No.6, 15 September 1990

The orientations required for channeling of the ion milled samples were all very similar and basically perpendicular to the surface of the samples. Unpatterned sections were also prepared for cross-sectional TEM by chemical thinning and low energy (2-3 ke V) iodine ion miHing. The cumulative ion dose received by the samples during this treatment was sig-nificantly less than during the Ar + ion milling and also was obviously occurring at right angles to the initial Ar mill treatment so that it does not influence what is observed in a cross-sectional micrograph. A TEOL 200CX microscope was used, and all micrographs were taken using multibeam bright-field imaging with seven beams included with the ob-jective aperture. The sample was tilted such that the beam direction was parallel to the [110] zone axis. This reduces any contrast effects at the surface and best allows one to see the surface topography.

RESULTS AND DISCUSSION The average mill rate normalized to the Ar f ion beam

current for InP and GaAs as a function of Ar T ion energy at 45° incidence angle and a temperature of 10 °C is shown in Fig. 1. Within experimental error ( ± 10%) in each case the etch rate depends linearly on the Ar t ion energy, with slopes of CAXmin· 1 XmA 1 xcm 2 XcV .. 1) and 0.8, respectively, for GaAs and InP. For GaAs this appears to be consistent with some previously reported datal.lb.17 in that a doubling of the ion energy leads to an increase in mill rate of For InP the mill rate increases more rapidly than for GaAs. There is very little data available for the sputtering yield of compound semiconductor targets, <) and

1000 Ar+, 500eV, 10"e

900 .. GoAs o lnP

N 800 E u ..... <{

r-O ..... E 700 - .,. ..... .- , c: .- \ 'E ".0 \ ." ..... 600 .... \

04 "" -- \ ... ..-... \ w \ !;;i 500 \ 0:: \ ::r: U 400 I-W

W 300 0:: W

200

100

0 10 20 30 40 50 60 80

ANGLE WITH RESPECT TO BEAM NORMAL

fIG. 2_ Average etch rate normali'£ed to ion <.:urrent for Ar' ion milling of InP and GaAs, m, a function of beam incidence angle measured from the normal.

Pearton et ai, 2761


for the case of InP one might expect preferential removal of P relative to the much heavier In atoms. Our measured mill rates correspond to sputtering yields for InP of 0.30 atoms per ion at 200-eV Ar f ion energy, 0.42 atoms per ion at 500 eV, and 0.80 at 800 keY when the Ar ion currents are fac-tored in. For GaAs the sputtering yield is 0.35 atoms per ion at 200 eV and 0.54 atoms per ion at 800-eV Ar + ion energy.

The experimentally observed mill rate dependence on the angle of incidence of the Ar ion beam for both materials at 500-eV Ar r ion energy is shown in Fig. 2. The mill rate increases with increasing incidence angle up to 60° where the mill rate is higher than at normal inci-dence. The mill rate then decreases for high beam incidence angles, as expected from linear cascade theory. q Our data for GaAs are consistent with the previously reported angular dependence. 1R For InP the sputtering yields were 0.36,0.45, 0.49, and 0.26 atoms per ion, respectively, at 0°,45",60°, and 75° angles of incidence, For GaAs the corresponding values were 0.31,0.33,0.39,0.44, and 0,23 atoms per ion at 0°, 15°, 45°, 60", and 75° incidence angles. Both materials show a similar angular dependence of the ion milling rate, with the InP removal rate being faster at all incidence angles for this Ar +- ion energy of 500 eV.

The surface morphologies of the ion milled materials were examined on patterned samples by SEM. Figure 3 shows results for InP samples, milled at 45° incidence angle at Ar l ion energies 0[200 e V (top left and bottom left), 500 eV (top right), and 800 eV (bottom right). The surfaces are reasonably smooth except in the latter case where preferen-tial sputtering of P during the ion milling treatment leads to In droplets remaining on the surface. In some cases, the In


spills over onto the unetched regions. The GaAs was not as sensitive to variations in the Ar + ion energy due to the much smaller mass difference between Ga and As relative to In and P. Results for 200 e V (top) and 500 e V (bottom) are shown in the SEM micrographs of Fig. 4, Even at 800-eV ion energy we observed relatively smooth morphologies on GaAs with no evidence of Ga droplets left on the surface. Due to mask irregularities, there are some regions of debris at the edge of the unetched bars.

A useful monitor of the introduction of near-surface damage into III-V semiconductors is the measurement of the total photoluminescent intensity from a sample before and after a particular treatment. 19 If deep level, nonradiative de-fects associated with lattice damage are introduced as a re-sult of a processing step then the band-to-band PL intensity from the sample will be reduced. Figure 5 shows the 300-K PL spectra from Fe-doped InP samples both before and after ion milling at different Ar ion energies. The beam was at 45° incidence angle for each treatment. The PL intensity is re-duced by approximately an order of magnitude after even the lowest ion energy (200 eV) milling treatment, and is further reduced for the higher ion energies (500 and 800 eV). These decreases are significantly greater than we have observed for reactive ion etching (RIE) of InP where the maximum ion energies are comparable to those used for the ion milling. 20 This is presumably because the etch rate is slower for ion milling relative to reactive ion etching in which there are both physical and chemical components to the etching. For ion milling therefore there is more accumu-lation of lattice disorder because this disordered material is not being removed as quickly as in RIE. The associated re-

FIG. 3. SEM micrographs of fea-tures ion milled into InP at 45 "C incidence angle of the beam with Ar' ion energies of 200 eV (top left and bottom left), 500 eV (top right-hand side) and SOD eV (bot-tom right-hand side).

Pearton et al. 2762


FIG. 4. SEM micrographs of features ion milled in GllAs at 200 eV (top) and 500 e V (bottom) Ar' ion energy. The heam incidence angle was 45".

duction in PL intensity is greater in ion milling as a result of the slower etch rates compared to plasma etching tech-niques. Similar results were obtained for ion milling of GaAs, as shown in Fig. 6, and we assume the same argument can be applied to that material as wen. It is interesting to note that annealing both materials at 700·C for 30 s produced

I.SC--------------,

>-t; 0.5 <n z W I-?;

1.35 1.40 1.45 ENERGY (eV)

lnP, woe, 45" - CONTROL --- 200V -'- soov ••• BOOV

1.5

FIG. 5. PL spectra recorded at 300 K of In!' ( Fe) ,amples ion milled using 200. 500, or 800-eV AI" ions.


1.0

, u w rn (/') I-Z :::> o u

"'00.5

>-l-(/') Z w I-

1.45 1.5

GaAs, 10·C, 45° -- CONTROL ••• 200V --- 500V -'- aoov

1.55 1.6 ENERGY (eV)

1.65

FIG. 6. PL spectl'a recorded at 300 K ofGaAs samples ion milled using 200, 500, or ROO-eV Ar' ions.

only moderate increases ( 20%) in the PL intensity. The annealing of the ion milling damage is treated in more detail later.

The introduction of deep levels associated with point defects in both InP and GaAs will cause carrier compensa-tion. In both materials the Fermi level is moved toward mid-gap with the presence of deep level centers and one can moni-tor the creation of such centers in the near-surface region using carrier profHing methods like capacitance-voltage. The presence of deep traps is manifested by a reduction in the carrier concentration near the surface. We were unable to perform C- V measurements on ion milled InP samples (n-type, 6X 1015 cm 3) because Hg probe contacts on these samples did not show any rectifying behavior after the mill-ing treatments. We have previously observed similar behav-ior on InP subjectcd to a variety of dry ctch processes. The exception has been when very low ion energies produced by Electron Cyclotron Resonance plasma etching were in-volved.20 Under such conditions, quite good Schottky con-tacts can be fabricated on the etched InP surface. As men-tioned above, annealing treatments following the ion milling were unsuccessful in restoring the surface quality to the point that we could measure diodelike behavior for our con-tacts.

For GaAs however, C- V measurements were possible under most conditions on the ion-milled samples. Figure 7 shows carrier concentration profiles from an n-type GaAs sample (No - N,j = 1.1 X 10\ 7 cm .\) both before and after ion milling for 5 min with 200-eV Ar t ions at 45° incidence angle. There are several interesting points to note. First, there is an apparent increa.se in the carrier concentra-tions near the surface where we cannot probe «,0.05 ,urn) because of the decrease in the zero-bia.s depletion region rela-tive to the control sample. This is consistent with the results of Colc et at.,:; who found the introduction of a highly con-ducting layer confined to the immediate surface following

Pearton et al. 2763


10 18

GaAs ION MILLED CONTROL

(I It 200V (45°) 400°C,30sec

o 0 500°C. 30 sec

1017 .. • 1"'1 ..

I (I

E \ .. { <.l \ (I I <i! \ I z \ .. I , \ I 0 , z " I

10 16 .... _./

0.05 0.1 0.i5 0.2 0.25 0.30 DEPTH (jLm)

FIG. 7. Apparent carrier profiles in IHype GaAs before and after 200-eV Ar' ion milling, and after subsequent annealing at 400 or 500 'C for 30 s.

low energy (500-eV) Ar t ion beam etching. This is coun-terintuitive in many respects, because damaged layers in GaAs tend to be highly resistive. The different behavior ob-served for ion milling maybe due to the creation ofa thin Ga-

10 19 .. GoAs ION MILL .. fI CONTROL fI .. \ .. .. SOOV (45") It \ 600"C, 30 sec .. \ i .. \ o 0 700"C! 30 sec i

10 18 .. \ \ f 0 • 0

r<l \ I 0 It I \ I E It " 0 u 0 <l: It 0

0 Z Q I 0

Z

10 17

1016L-__ -L ____ L-__ -L __ ____ __ __

0.05 0.1 0.i5 0.2 0.25 0.3 DEPTH (f-/-m)

FIG. 8. Apparent carrier profiles in II-type GaAs before and after SOO-eV Arion milling, and after subsequent annealing at 600 or 700 'C for 30 s.

2764 J. Aopl. Phys., Vol. 68, No.6, 15 September 1990

rich layer because of preferential sputtering effects. The sec-ond point of interest is that beneath the apparent increase in carrier concentration there is region where there is a reduc-tion in net n-typcness of the GaAs. This is the more usually observed result for ion bombardment of III-V materials, with the charge carriers being compensated by midgap ac-ceptors associated with lattice disorder. The third point of interest is that relatively low-temperature annealing treat-ments restore the electrical characteristics of the ion-milled GaAs. A 400 °e, 30 s treatment initially causes a worsening of the carrier profile in the region 0.1-0.15 fim from the surface and the zero-bias depletion depth is also reduced, indicating that the near-surface conducting layer becomes worse< However, annealing at 500°C for 30 s basically re-stores completely the initial carrier profile, although the zero-bias depletion depth is still slightly smaller than in the control sample.

The features observed for low energy (200 e V) Ar + ion milling were even more prevalent for higher ion energies. Figure 8 shows apparent carrier combination profiles in n-type GaAs (N D - NA = 1.1 X 1017 cm 3 ) before and after milling with a 500-eV Ar + beam at 45° inci.dence angle. For this higher ion energy the highly conductive, near-surface layer is now evident in the carrier profiles. With annealing at progressively higher temperatures this near-surface feature is reduced in concentration and indeed the carrier density in

4 E

100 InP - 500\1 (45°) -'- 800\1 (45°) - - 500V (0°)

01------

100

o BIAS VOLTAGE (V)

InP ION - MILLED 500\1

- 390A REMOVED

o BIAS VOLTAGE (V)

FIG. 9. J- V data from Au-hlP Schottky diodes fabricated on ion milled, n-type InP (top) and after removal of 390 or 945 A of material prior to evapo-ration of the Au contact (bottom).

Pearton et al. 2764


the region up to 0.1 pm must be lower than in the control sample after annealing at 700 °C because of the increased zero-bias depletion region. This again is consistent with past results showing that ion etching at low energies generates a thin conductive layer, whereas high energy ion beam etching creates a thicker, higher resistance layer. K wan et ai. 7 found that damage introduced by low energy (<,100 keY) ions could be substantially removed by annealing by °c, whereas GaAs damaged at >400 eV actually showed a deg-radation in quality with such annealing. Further, it was found that the zero-bias capacitance of diodes was increased by ion beam damage up to eV, and thereafter was reduced for energies up to 1000 eV. We monitored the phase angle of the signal during the C- V measurements and reject-ed data where this angle was less than 80° (an ideal capacitor should have 90°). A decreasing phase angle is an indication of either a poor quality rectifying contact or the presence of highly conducting or highly resistive near-surface layers. The resulting equivalent circuit for ion etched GaAs has been modeled previously.s The donorlike states which give rise to the increase in effective carrier concentration near the surface presumably result from native defects or complexes thereof which are removed by annealing at progressively higher temperatures. These states are in competition with deep acceptor states which tend to reduce the n-typeness of the GaAs.

As mentioned above, ion milling of InP led to surfaces to which we could not fabricate rectifying contacts. Both Hg and evaporated Au metallizations on milled InP samples showed ohmic behavior, as evidenced by the J- V characteris-tics of Fig. 9. Similar data were observed for all ion energies (200-800 eV) and angles of incidence (0-75" from the verti-cal). The damaged, ncar-surface region was progressively removed by wet chemical etching in IHCI:5HzO and the etch depth directly measured by Dektak stylus profilometry after masking a section of the sample with apiezon wax. E-vaporated Au Schottky contacts were then deposited onto the etched surface. Figure 9 also shows 1- V characteristics from samples from which 390 or 945 A were removed prior to deposition of the Schottky contact. Removal of 390 A of material was not sufficient to restore the 1- V characteristics, while after taking off945 A, the J- V data was identical to that of a control sample. It was found that for 500-eV Ar' ion milling at least 600 A ofInP had to be removed hy wet chem-ical etching to restore the initial electrical characteristics of the material as evidence by the /- V data.

Forward and reverse 1- V characteristics for TiPtAu Schottky contacts on ion-milled GaAs amples are shown in Fig. 10. The forward voltage parts of the curves cannot be explained by pure t.hermionic emission and show that a large amount of defect-assisted tunneling is contributing to the current conduction in the structures. The reverse character-istics showed increased currents when high Ar ion ener-gies were used during the milling treatments. It also appears that vertical incidence ion milling creates more damage than 45° beam angle. This is presumably also related to the slower etch rate at vertical incidence, allowing more accumulation of damage.

Annealing the ion-milled (500 eV, 45") GaAs even at

2765 J. AppL Phys., Vol. 68, No.6, 15 September 1990

-----GoAs lON-MillED _.- 200V (45") - 500V (45tl

)

800V (45") ® $ 500V (O")

0.4 VF (VOLTS)

Go As ION - MillED

-.-

-3

0_.- 0_.- .-.

VR (VOLTS)

0.8

.....

o

f'JG. 10. I'orward (top) and reverse (bottom) I-V characteristics from TiPtAu Schottky dJodes fabricated Oil ion-milled II-type GaAs.

700°C for 30 s brought little improvement in the J- V charac-teristics, as described earlier in connection with the PL data, and as shown in Fig. 11. The diode is still very leaky with a high reverse current and nonideal forward characteristic. This is also consistent with previous results indicating that annealing up to 400°C actually leads to a deterioration in the 1- V characteristics of diodes fabricated on ion-milled GaAs. 7 Our results show that even 700°C annealing is insuf-ficient to remove the ion-milling damage. We also performed a serics of cxperimen ts in which various thicknesses of mate-rial were removed by wet chemical etching in a IH,SO,,: 1 H 2 0: lOH2 0 solution prior to evaporation of the Schottky contacts. Removal of - 235 A of material from the surface of a sample ion milled with 500-cV Ar j ions for 4 min at 45" incidence angle produced a substantial improve-ment in diode characteristics. We found that under these conditions a minimum of - 485 A had to be etched offhefore the J- V characteristics were similar to those of a control sam-ple. This is consistent with other reports of damage depth. 4.R. 1 D,lO

Previously it has commonly been assumed that ion mill-ing of H[-V materials leads to the formation of a shallow amorphous layer in the near-surface region. 1,1,0.7 Surprising-ly this is stated without supporting data and a close exam ina-

Pearton et al. 2765


tion of publications dealing with crystalline quality after ion beam etching actually show no evidence for amorphous lay-er creation. 8,10.20 We examined a variety of InP and GaAs samples after ion milling and after subsequent annealing, Figure 12 shows ion channeling spectra from InP ion milled for 4 min at either 200 or 800 e V (45° incidence angle), and also from the latter sample after an 800°C, lOs anneal. These spectra were recorded by the 1600 detector. We use both the minimum backscattering yield (X min) and the areal density calculated from the surface peaks in the lOY spectra as indi-cators of the crystalline quality after ion milling. The Xll1in is defined as the height in channeled spectrum divided by the height in the rotating random spectrum at the same channel number. In this case the height was measured at the point where the channeled spectrum was at a minimum. Due to the differences in depth resolution, the X mill values obtained at 105° and 160° should not be compared directly to each other. The areal densities give an idea of the number of dis-placed surface atoms. For InP only the In backscattering peak was measured, while for GaAs both the Ga and As peaks were measured due to the close proximity of the masses ofGa and As. The InP sample ion milIed at 200 eV had a X min of2.9% at 105· and 3.1 % at 160°, similar to that of an unetched control sample. The sample etched with an 800

u. ...... GoAs ION-MILLED " e 700"C,10sec

" - - 235 A REMOVED o - 485A REMOVED

10 -10 L--L--'-_.L---l..._.L--L.--'_-'----'---'

II: .....

o 0.4 VF (V)

0.8

10-1 r-----------------,------. "e GoAs ION-MILLED

--- --- --

* .. $ ..

e

" " .. ..

____ ____ __ ____ -5 -4 -3 -2

VR (R) -1 o

FIG. 11. Forward (top) and reverse (bottom) /- V characteristics from TiPtAu Schottky diodes on ion-milled GaAs that had either been annealed (7()() "C, 1 () s) or wet etched to remove 235 or 485 A of material prior to deposition of the contact.

2766 J, Appl. Phys" Vol. 68, No.6, 15 September 1990

25

en r z ::; 8 15

'" S2 :; 10 w >=

5

o

4He 2.275 MeV InP 160'

ION MILL 200V I ION MILL aoov I ION MILL 800V+8OO"/l0s

I RANDOM

., -._.'

200 400 CHANNEL NUMBER

FIG. 12. Ion channeling spectra recorded at 160' detection angle from ion milled InP before and after annealing.

eV beam showed a significant level of damage in the first 200 A, with a Xl11in of 9.1 % behind this damaged region. The crystalline quality of this sample actually became worse after a 500 °ellO s anneal with a Xmin of 13.4% measured in the 105° spectrum. This is consistent with other reports using electrical characterization techniques. 7,21 Annealing at 800 ·C does produce a substantial reduction in gross lattice disorder, with the Xmin at 105° returning to 5.3%. The In surface peak returned to 1.9 X 1016 em - 2 compared to a value of 1.1 X 1016 cm - 2 for an unetched control sample. The changes in the near-surface damage peaks are more clearly seen in the 105° ion channeling spectra of Fig. 13. A compilation of the ion channeling data is given in Table I. The sample annealed at 800·C for 10 s has less crystal dam-age in the upper 100 A than does the sample ion milled at 200 e V, but the disorder peak after annealing extends to 800 A. The damaged region after 500 ·C/IO s annealing of the 500-e V milled sample also extends to greater depths than before annealing, indicating a migration of the disorder.

10

In I-Z ::;) 0 u

'" S2 5

0 .J W >-

o

4He 2.275 MeV InP 105"

ION MILL 200V ION MILL BOOV

- 800V+ 500·C, 30sat o 0 800V + 800·C, 30sec

200 CHANNEL NUMBER

400

FIG. lJ, Ion channeling spectra recorded at 105" detection angle from ion milled InP before and after annealing.

Pearton et al. 2766


o ..J '!d >- 5

o 200

4 He 2.275 MeV GaAs 1600

ION MILL 200V ION MILL BOOV

400 CHANNEL NUMBER

FIG. 14. Ion channeling spectra recorded at 160" detection angle from GaAs ion milled with 200 or 800-eV Ar' ions.

Similar data were obtained for GaAs samples ion milled at 200 or 800 eV and subsequently annealed at either 500 or 800°C for lOs. The ion channeling spectra obtained at 160° are shown in Fig. 14. The Xmin values were 4.9% for the 200 eV sample and 5.5% for the 800 eV sample. These results indicate that GaAs is considerably more resistant to the in-troduction of lattice disorder than InP ion milled under the same conditions. After annealing at 500°C for 10 s, the 800 eV sample has a Xmin at 1600 of 7.0%. This again is worse than in the as-milled materiaL Annealing at 800 "C does im-prove theXmin value to 5.2%, with a reduction in the surface peak from 4,1 X 1016 em 2 in the as-milled GaAs to 2.3 X 1016 cm - 2 after the 800 DC anneaL These values are tabulated in Table I. The ion channeling data obtained at 105° are shown in Fig. 15.

The subsurface defects and surface topography of both InP and GaAs samples ion milled at 500 eV (45° incidence angle) were examined by cross-sectional TEM. Micro-

in I-Z :::> 0

4He 2.275 MeV GaAs

\ u

'" 5 1050

Cl ION MILL 200V

\ .J ION MILL 800V \ w 800V + 500°C, 30 sec ;;:

200 400 CHANNEL NUMBER

FIG. IS. Ion channeling spectra at !O5' detection angle from ion mllled GaAs before and after annealing.

2767 J. Appl. Phys., Vol. 68, No.6, 15 September i 990

1\" Milled Anne3!cd

GaAs

IqP

FIG. Iii. TEM micrographs taken under hrtght field conditions from InP and GaAs ion milled at SO() eV, 45° incidence angle, and after annealing at SC,() 'C, J() s. A hand of dislocation is present in the annealed GaAs.

graphs taken under bright Held (g22o, S > O) conditions are shown in Fig. 16, The annealing for both materials was per-formed at 800°C for 10 s, The GaAs showed a layer of ex-tended defects that formed upon annealingo These consisted of a relatively high concentration (> 10'0 cm 2) of disloca-tion loops (diameter 60-70 A) at a depth between 130 and 460 A below the surface, These are observed as small dark regions, This depth is much greater than the mean range of 500-e V Ar + ions in GaAs (24 A, with a straggle of 12 A) as determined hy a Monte Carlo simulation, The fact that the visible disorder is located deeper than the projected range of the impinging ions is a typical feature of dry etch pro-cesses, 15 0 I'loll It is important to remember that the electrical measurements indicate much greater incorporation depths for point defects, rather than the extended defects accessible to TEM. No dislocation loops were observed upon annealing the ion-milled InP,

The InP samples had rougher surface morphologies than comparably ion-milled GaAs, as shown in the (110) cross-section TEM micrograph of Fig, 17 obtained in the OIl-

axis condition with seven beams within the objective aper-ture. The InP displays some surface structure with a peak-to-valley height of -170 A for the 500-eVion milled sample. This is consistent with the SEM data reported earlier. The GaAs samples both before and after 800°C, 10 s annealing showed smooth morphologies even on a microscopic scale.

Pearton et al. 2767


As Milled Annealed

GaAs

InP

FIG. 17. TEM micrographs takcE under conditions to highlight the surface morphohJgy of ion milled (500 cV, 45' incidence angle) Inl' and GaAs be-fore and after annealing at SUO T, 10 s.

SUMMARY Taken together, our data on ion-milling disorder in JnP

and GaAs illustrate the following key points: (i) For low energies ( 200 e V) there is introduction of

a very near-surface conductive layer ill GaAs, with deep ac-ceptor traps at greater depths leading to carrier compensa-tion. Both types of defects can be removed by annealing at

500 0c. (ii) For higher ion energies (> 500 eV) the near-surface

conductive layer is more prominent, and even annealing at 700 "C does not restore the initial characteristics ofthe mate-rial. Annealing at lower temperatures may actually lead to a deterioration of the crystalline quality.

(iii) For 500-eV Ar I ion milling at 45° incidence angle, at least 600 A has to be removed by wet chemical etching of InP and - 485 A from GaAs before the initial diode charac-teristics are restored.


(iv) There is no amorphous layer formation under our ion milling conditions. Dislocations form in GaAs after an-nealing, but they are not observed in InP. Ion channeling detects more lattice disorder in InP milled under the same conditions in comparison to GaAs.

A similar picture for ion milling damage emcrgcs as was postulated for dry etch damage in InP.15 For 500-eV ion milling gross damage is detectable by ion channeling to depths of -·200 A. Beyond this is a region containing point defects which alter the carrier concentration to depths of

1000 A. Migration of near-surface ( - 200 A) defects oc-curs during annealing, and ion channeling shows a broader disordered region. In the as-milled condition, removal of 500-600 A by wet chemical etching is enough to essentially restore the electrical characteristics of the material because this takes off the grossly disordered region and a section of the electrically compensated layer. The remaining region where there is carrier compensation is then totally within the zero bias depletion depth, and has little effect on the J- V characteristics of simple diodes.

I See for example, e. I. H. Ashby, Properties of GaAs, EM IS Datareview, RN 15422, (lEE, London, 1985).

2M. Kawabe, N. Kanzaki, K. Masuda, and S. Namba, App!. Opt. 17,2556 ( 1978).

'S. W. Pang and W. J. Piancentini, J. Vac. Sci. Techno!. B 1, 1334 (1983). 'Y. Yuba, T. Ishida, K. Garno, andS. Namba,J. Vuc. Sci. Techno!. B6, 253

(1988). sE. D. Cole, S. Sen, and L. C. Burton, J. Electron. Mater. 18, 527 (1989). 'P. J. Smith and D. A. Allan, Vacuum 34, 209 (1984). 7p. Kwan, K. N. Bhat, J. M. Borrego, and S. K. Ghandi, Solid-State Elec-tron 26, ! 25 (1983).

"Y. G. Wang and S. Ashok, Nucl.lnstrum. Methods Phys. Res. B 39,461 ( 1989).

"Po C. Zalm, Vacuum 36, 787 (1986). ;('S. W. Pang, M. W. Geis, N. Ercfemow, and G. A. Lincoln, J. Vac. Sci.

Techno!. B 3, 898 (1985). ! 'M. Syzmonski and R. S. Bhattacharya, App!. Phys. 20, 207 (1979). !lG. Bctz and G. K. Wehner, Sputtering by Particle Bombardment, edited

by R. Behrisch (Springer, Berlin, 1983), Vol. 2, p. 1 L "A. 5akalas and S. Zhukauskas, Solid State CommuTI. 7U, 363 (1989). 14y' Yuba, K. Garno, Y. Judai, and S. Mamba, The PhysicsojVLSJ, edited

by J. C. Knights (AlP Conf. Ser. Proc., New York, 1984), pp. 286-290. lOS. J. Pearton, U. K. Chakrabarti, and F. A. Baiocchi, App\. Phys. Lett. 55,

1633 (1989). I be. M. Melliar-Smith and C. J. Mogab, Thin Film Processes, edited by J. L.

Vossen and W. Kern (Academic, New York, 1978), p. 497. 17). W. Coburn and H. F. Winters, J. Vac. Sci. Techno!. 16, 391 (1979). '"So Somekh and H. C. Casey, Jr., Appl. Opt. 16,126 (1977). lOR. Germann, A. Forbel, M. Brcsch, and H. P. Meier, J. Vac. Sci. Techno!.

B 7,1475 (1989). 2('S. J. Pearlon, U. K. Chakrabarti, A. P. Kinsella, D. Johnson, and C. Con-

stantine, Appl. Phys. Lett. 56,1424 (1990). "5. K. Ghandi, P. Kwan, K. N. Bhat, and J. M. Borrego, IEEE Electron.

Rev. Lett. EDL-3, 50 (1982). ?JH. F. Wong, D. L. Green, T. Y. Liu, D. G. Lishan, M. Bellis, E. L. Hu, P.

M. Petroff, P. O. Holtz, and I. L. Merz, J. Vac. Sci. Techno!. B 6, 1906 (1988).

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ion milling damage in inp and gaas s. j. pearton, u. k....

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