Study of temperature dependent hydrogenation on nearsurface strained quantum wellsYingLan Chang, Mohan Krishnamurthy, IHsing Tan, Evelyn L. Hu, James L. Merz, Pierre M. Petroff, A. Frova,and V. Emiliani Citation: Journal of Vacuum Science & Technology B 11, 1702 (1993); doi: 10.1116/1.586508 View online: http://dx.doi.org/10.1116/1.586508 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/11/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Silicon interlayer based surface passivation of nearsurface quantum wells J. Vac. Sci. Technol. B 13, 1794 (1995); 10.1116/1.587814 Study of surface stoichiometry and luminescence efficiency of nearsurface quantum wells treated by hydrogenions and atomic hydrogen J. Vac. Sci. Technol. B 12, 2605 (1994); 10.1116/1.587217 Study of hydrogenation on nearsurface strained and unstrained quantum wells J. Appl. Phys. 75, 3040 (1994); 10.1063/1.356150 Reduced quantum efficiency of a nearsurface quantum well J. Appl. Phys. 74, 5144 (1993); 10.1063/1.354276 Luminescence efficiency of nearsurface quantum wells before and after iongun hydrogenation Appl. Phys. Lett. 62, 2697 (1993); 10.1063/1.109235

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Study of temperature dependent hydrogenation on near-surface strained quantum wells

Ying-Lan Chang, Mohan Krishnamurthy, I-Hsing Tan, Evelyn L. Hu, James L. Merz, and Pierre M. Petroff Center for Quantized Electronic Structure (QUEST), University of California, Santa Barbara, California 93106

A. Frova and V. Emiliani Dipartimento di Fisica, Universitd di Roma "La Sapienza," Roma 00185, Italy

(Received 25 January 1993; accepted 25 March 1993)

The incorporation of hydrogen by ion-gun irradiation into near-surface and deeply embedded InO.13Gao.87As/GaAs strained quantum wells (QWs) has been studied by photoluminescence spectroscopy and transmission electron microscopy (TEM). Degradation in the free exciton luminescence and the appearance of hydrogen-related shallow or deep states have been observed within the near-surface QW after hydrogenation. This effect is more pronounced, the higher the hydrogen dose. In contrast, the deeply embedded QW is only slightly affected by the hydrogenation process even at high substrate temperature and hydrogen ion dose. TEM reveals hydrogen-induced plateletlike structure in the vicinity of the near-surface QW and of the GaAs buffer layer/GaAs substrate interface after room temperature and high temperature (250 ·C) hydrogenation, respectively, which ascertain the extension and nature of the hydrogen-enriched regions throughout the whole material structure.

I. INTRODUCTION

The passivation of deep defects and shallow impurities in bulk semiconductors by hydrogen has been extensively studied in recent years. 1

•2 It is generally believed that hy­

. drogen readily binds to broken chemical bonds in semicon­ductors, reduces the electrical activity of a defect, thereby leading to the enhancement of carrier lifetime and lumi­nescence efficiency.3 We have previously reported that hy­drogen can passivate an oxide-covered AIGaAs surface.4 A complete recovery or even enhancement of luminescence efficiency was observed in a near-surface GaAs/ AIGaAs quantum well (QW) after low-energy ion-beam hydroge­nation, already at room temperature.

The behavior of hydrogen and its effectiveness in passi­vation of semiconductors, strongly depends on the materi­als and conditions of passivation. We have reported in pre­liminary form that a near-surface QW, made by InGaAs/ GaAs does not show recovery, but rather stronger degradation.4 Neethling et al. have reported the formation of hydrogen platelets in high energy (300 keY) proton­bombarded GaAs samples.5

-7 However, it has also been

demonstrated that hydrogenation carried out at heavy doses can induce microdefects and electronic deep levels in single-crystal silicon, which are unrelated to either plasma or radiation damage.8 A {11 I} oriented platelet structure whose density is proportional to the near-surface concen­tration of hydrogen was observed by transmission electron microscopy (TEM).8 The appearance of the Si-H bonds, revealed by Raman spectroscopy, indicated that the plate­lets were not only induced by hydrogenation but also sta­bilized by the direct incorporation of hydrogen into the defect structure.

Recently, the observation of optical emission from H-related complexes after high temperature hydrogenation

in III-V semiconductors, e.g., molecular-beam epitaxy (MBE) grown bulk GaAs and AIGaAs, and InGaAs/ GaAs strained QWs has been reported.9 The incorporation of hydrogen opens an alternative and competitive recom­bination channel via either shallow or deep radiative states, and therefore reduces the luminescence efficiency of the free excitons (FEs). In this work, we carried out a study of hydrogenation, for different substrate temperatures and ion doses, on the near-surface and deeply embedded Ino.13Gao.87As/GaAs strained QWs. Using photolumines­cence (PL) spectroscopy and TEM, we have been able to study the role of compressive strain inside the wells, and the effects of substrate temperature and irradiated-ion dose in modifying the diffusion profile of hydrogen. Our low­temperature PL spectra revealed the nature of the hydrogen-induced active defects, e.g., shallow or deep level corresponding to different passivation conditions, and al­lowed us to evaluate the effect of the local hydrogen con­centration on the formation of hydrogen-induced defects. Furthermore, the TEM micrograph of the room­temperature-treated sample provides evidence of possible hydrogen pile up at the immediate surface region through the appearance of defect structures. The TEM micrograph of the high-temperature hydrogenated sample, however demonstrates the accumulation of hydrogen related defects at the GaAs buffer layer/GaAs substrate interface. These structural changes can be correlated with the spectral changes brought about by the differing conditions of hy­drogenation.

II. EXPERIMENT

The sample structure used for this study is shown in Fig. 1, and is comprised of a near-surface and a deeply situated Ino.l3Gao.87As/GaAs QW grown on a noninten-

1702 J. Vac. Sci. Technol. B 11(4), JullAug 1993 0734-211X/93/11(4)/1702/4/$1.00 @1993 American Vacuum Society· 1702

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1703 Chang et sl.: Temperature dependent hydrogenation

GaAs35 nm ~'--'-':"";""'-"-'i'< InGaAs 8 nm (Q1)

GaAs350 nm

... ',... .' ... ,... InGaAs 6 nm (Q2)

GaAs buffer 150 nm

lnm AJAsil nm GaAs, 27 periods

GaAs buffer 100 nm

GaAs substrate

FIG. 1. Sample structure.

tionally doped GaAs (100) substrate by MBE. The upper QW denoted by Ql, 8 nm in width, is separated from the surface by a 35 nm thick GaAs layer (designated as a "surface barrier layer"), while the lower 6 nm QW de­noted by Q2 is separated by a 350 nm GaAs layer from the upper QW, and by a GaAs buffer layer and GaAs/ AlAs supedattices from the GaAs substrate. The hydrogenation was carried out with a Kaufman ion source,1O operated at 100 eV with a beam diameter of -1 in. The current density was between 1 and 25 j.tA/cm2

• The background pressure of the chamber was _10-6 mbar and the total pressure, after introduction of H, -4 X 10-4 mbar, with a H-ion flow of - 60 sccm. After the ion flux was shut off, the sample remained at the operating temperature for 30 min in the residual H ambient, then it was cooled down. The total H-ion dose reaching the sample surface can be mon­itored through the current reading and carefully repro­duced (this dose is of course much larger than the actual number of incorporated particles). Under these conditions, bombardment damage can be estimated not to extend be­yond 10 nm below the surface, while the H depth profile and volume density are limited by diffusion, a thermally activated process. PL spectra of the near-surface QW were taken at 1.4 K by a Ge detector, using -0.5 W /cm2 exci­tation from the 488 nm line of an Ar-ion laser.

III. RESULTS

Figure 2 shows the PL spectra for the samples hydro­genated at room temperature with different ion doses. Curve (a) of Fig. 2 displays the PL spectrum obtained from a virgin sample, where the lower energy peak denoted as Q1 at 1.404 eV is from the near-surface QW and the other one denoted as Q2 at 1.427 eV is from the deep QW. Each excitonic peak corresponds to the fundamental tran­sition between electron and hole states (e1-hhl). A slight increase of the luminescence of Q 1 was observed after hy­drogenation with dose [H]=1016 cm-2

, as shown in curve (b). However, an increased dose of 1018 cm-2 produced a

J. Vac. Sci. Technol. a, Vol. 11, No.4, Jul/Aug 1993

:>.. .... .... til

Ql Q2

(a)

~ ~(~b~) ______________________ ~

.... . : ,.J I.l(~c) ...... _.,..... Q.;

1.15 1.2 1.25 1.3 1.35

Energy (eV) 1.4 1.45

1703

FIG. 2. PL spectra after room temperature hydrogenation; (a) virgin sample, (b) [H) = 1016 cm-2

, and (c) [H]= 1018 cm-2•

deep H-related broad band peaking at -1.273 eV, accom­panied by a remarkable decrease in luminescence intensity of Q 1. This is shown in curve (c) of Fig. 2. In parallel, Q 1 exhibited a -2 meV redshift, relative to the energy peak position of the virgin sample. Both the peak intensity and the energy position of Q2 in curve (c) remain quite the same as in the virgin sample. At room temperature, be­cause of the insufficient thermal activation, H can be ac­cumulated only near the surface, so that the near-surface QW can be used to probe the competitive effect of the H-induced deep levels with respect to the el-hhl exciton recombination.

Figure 3 shows the PL spectra obtained from samples exposed to different H ion doses at high temperature (T = 250°C). Curve (a) of Fig. 3 is for the virgin sample. In this case of high-temperature hydrogenation, we expect that hydrogen can be thermally activated to diffuse further into the material. At a low dose of [H] = 1016 cm -2, no significant change in either peak position or PL intensity was found in curve (b) with respect to that of curve (a). However when the hydrogen dose is greater than _1017

cm-2, a H-related shallow bound state, -14.5 meV below

the e I-hh 1 line of Q 1 was observed, with no change for the

,.-.. ::; (a) o:i '-'

.t> (b) ·in c ~ ..... (c) .:

...::I ~ (d)

1.35 1.37 1.39 1.41 1.43 1.45

Energy (eV)

FIG. 3. PL spectra after high temperature (T=250 ·C) hydrogenation; (a) virgin sample, (b) [H]= 1016 cm- 2, (c) [H) = 1017 cm-2, and (d) [H]=1018 cm- 2•

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1704 Chang et 81.: Temperature dependent hydrogenation

~Ql

~Q2

(a) (b)

FIG. 4. Dark-field (200) cross-sectional TEM of the near-surface QW (Ql) and deep QW (Q2). (a) A mottled contrast is observed in the barrier regions near Q 1 indicating formation of possible hydrogen related precipitates or platelets after room temperature hydrogenation at 1018

cm- 2• (b) The TEM image shows no defect structure near Ql in high­temperature hydrogenated sample. The mottled contrast in the surface region is tentatively assigned to be from hydrogen-induced platelets.

Q2 line. Curves (c) and (d) of Fig. 3 display the spectra for hydrogenated samples corresponding to [H]= 1017 and 1018 cm -2, respectively. At the dose of 1018 cm -2, the emission of H-related shallow-bound state becomes domi­nant. A smal1loss in luminescence ( - 20%) was observed for Q2 under the highest H-dose condition. As the excitation-power dependence of PL indicates a faster satu­ration in luminescence intensity for the shallow-bound ra­diative state than for the FE in Q1, we conclude that the former is associated with the formation of a new H-related level, presumably a donor.9

The appearance of a defect structure after room­temperature hydrogenation, with [H]= 1018 cm- 2, has been observed by TEM. In Fig. 4(a), we show a dark-field cross-sectional TEM micrograph viewed by the (OIl) pro­jection. The image reveals a mottled structure within - 50 nm of the surface except in the near-surface QW; each defect has an average diameter of - 7 nm. Such a defect structure is not apparent in the near-surface region of the high-temperature hydrogenated sample, as shown in Fig. 4(b). We tentatively assign these defects to be a result of the initial stages of hydrogen-induced platelet formation.

Figure 5 shows the TEM of samples hydrogenated at high temperature (T=250 ·C) with [H]= 1018 cm-2. The mottled structure is no longer apparent in the near-surface region, indicating a much lower local hydrogen concentra­tion, compared with the sample passivated at room tem­perature. Instead, a {1I1} oriented plateletlike structure exists at the interface between the GaAs buffer layer and the GaAs substrate, indicating an accumulation of hydro­gen in this area. The average size of these platelets is of the order of - 100 nm.

IV. DISCUSSION

A deep and broad band of radiation similar to curve (c) of Fig. 2 has been observed in MBE-grown GaAs on the semi-insulating GaAs substrate. 9 By comparing the

J. Vac. Sci. Techno •• a, Vol. 11, No.4, Jul/Aug 1993

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_Q2

GaAs buffer

GaAs substrate

FIG. 5. Bright-field TEM image of high-temperature hydrogenated sam­ple. The SUbstrate-buffer layer interface position is shown. Large hydrogen-induced defect structures are shown. Some of these defects show a line of contrast which is always oriented along the (111) direction, as indicated by the arrow in the image, irrespective of the diffraction vector used. These defects are likely to be planar or disklike lying on a {111} plane.

InGaAs/GaAs mUltiple QWs grown on a GaAs substrate to those grown on a Si substrate, it has been shown that such H-related bands are stronger if there is a greater abundance of defects present, which allow a more effective incorporation of hydrogen. l1 ,12 Moreover, the low hydro­gen diffusion at room temperature causes its density to be locally very high, so that a number of H atoms can have access to vacancy sites. As a consequence, a configuration for the H-donor/Ga-vacancy complex, with the H donor in bond-center and H atoms passivating dangling bonds, can be the source of the deep radiative band at -1.273 eV, as already proposed for bulk MBE GaAS.9

,13 The decrease of the FE emission in the QW is therefore due to escape of carriers, via tunneling, to the highly hydrogenated region of the GaAs barrier, followed by vacancy/donor radiative recombination.

The possibility of the same kind of recombination inside the well, however, cannot be ruled out. The incorporation of hydrogen in the QW would lead to a volume dilation, which has been estimated to be -0.01 %, corresponding to the energy shift of the FE line that we have observed. However, it has been recently demonstrated for bulk GaAs that, whenever there is hydrogen accumulated at surfaces or interfaces, an electric field develops which shifts the FE line by the correct quantitative amount.9,14 In the present case, both mechanisms are likely to be operative.

The hydrogen pile-up effect within the subsurface zone of the room-temperature treated sample is clearly evident from the TEM micrograph. The mottled structure in Fig. 4(a) shows that hydrogen is distributed within -50 nm from the surface, with the exception of the well region, indicating a lower hydrogen density compared with the barrier region. The InGaAs material, whose bulk-lattice constant is greater than that of GaAs, experiences com­pressive strain during growth. The TEM micrograph sug­gests that hydrogen reaching the well region may prefer diffusing into the barrier to lower the system strain energy. By performing room-temperature hydrogenation, we can decouple thermal effects and identify the role of strain in modifying the distribution profile of hydrogen.

For high-temperature hydrogenation, a deeper diffusion

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1705 Chang et sl.: Temperature dependent hydrogenation

of hydrogen into the material can be thermally activated, therefore producing a lower local hydrogen concentration in the vicinity of the near-surface QW. The density of do­nor states generated within the near-surface region and QW is too low to create a deep band. This has been con­firmed by the TEM micrograph, indicating no platelet structure close to the upper QW region. In this situation, a shallow transition with a peak energy 14.5 meV below that of FE line can be observed. Because of the dependence of such energy on well width,12 the corresponding level has been associated with a H-related donor situated in the well or at its surrounding interface.

Beside the factors of hydrogenation temperature and ion dose, the volume incorporation of hydrogen is also affected by the density of impurities and defects where hydrogen can bind. A clear example is seen in the present structure, where TEM (Fig. 5) shows large defect structure which is possibly related to the high concentration of hydrogen at the GaAs buffer layer/GaAs substrate interface. Prelimi­nary analysis of the large defects indicates that they may be different from the types of defects introduced by high en­ergy proton implantation reported by Neethling et al.5-7 The strain field of these defects appears insensitive to the diffraction vector under two beam conditions. However, a line of contrast appears in some defects oriented along the ( 111 > direction. We surmise that these defects are planar or disklike, lie on a {Ill} plane and are likely to the pre­cipitates formed as a result of reactions between GaAs and hydrogen.

v. CONCLUSION

Temperature and ion-dose dependent hydrogenation has been carried out on InGaAs/GaAs strained QW struc­ture. Room-temperature, high-dose hydrogenation has produced high near-surface hydrogen concentration, a lo­cal hydrogen-related defect structure, accompanied by a drop in the FE emission from the QW, and the activation of a deep Ga-vacancy-related emission band. These fea­tures account for the failure to achieve recovery of lumi­nescence in the near-surface QWs in the present experi­ment. Nevertheless, a slight increase of FE luminescence in the near-surface QW, which should be related to the pas­sivation of surface states, has been achieved by using low­dose, room-temperature hydrogenation. The hydrogen dis-

J. Vac. Sci. Technol. B, Vol. 11, No.4, Jul/Aug 1993

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tribution profile after high-temperature treatment is entirely different. A shallow H-related state, exhibiting a binding energy of -14.5 meV, becomes dominant at large ion dose. TEM has shown large defects indicative of the accumulation of hydrogen at the GaAs buffer-layer/GaAs­substrate interface after high temperature, high dose hy­drogenation, which is probably due to native defects or contamination in this region. This is the first report of the formation of {lll} oriented plateletlike structure and their behavior during thermal treatment in the InGaAs/GaAs QW samples after hydrogenation. Finally, we have also detected the effects of strain and defects on the migration process of hydrogen, following the formation of H-related radiative states and defect structures.

ACKNOWLEDGMENT

This work was supported by the NSF Science and Tech­nology Center for Quantized Electronic Structures (QUEST), Grant No. DMR 91-20007.

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