Optically induced photomemory by 1 to 1.35 eV photons in the near-intrinsic spectral region on semiinsulating bulk GaAs

J. JIMENEZ et al. : Optically Induced Photomemory on Semiinsulating GaAs

phys. stat. sol. (a) 87,623 (1985)

623

Subject classification: 16; 13.4; 22.2.1

Laboratorio F h i m del Entado ndlido, Facdtad de Ciencian, Vallaadolidl)

Optically Induced Photomemory by 1 to 1.36 eV Photons in the Near-Intrinsic Spectral Region on Semiinsulating Bulk GaAs BY J. JIMENEZ, J. BONNAFE~), P. HERNANDEZ, and J. A. DE SAJA

It is well known that photons with energies ranging from 1 to 1.35 eV induce strong transformations on the photocurrent response of semiinsulating bulk GaAs. There are two photomemory effects which are strongly dependent on the sample kind; so there are samples that - exhibit an enhancement of the 1 to 1.35 eV extrinsic photocurrent after sufficiently long excitation below 120 K, whereas other samples show optical quenching of the photocurrent under the same excitation conditions. These phenomena can be understood on the basis of strong changes on the configuration of deep traps under illumination with photons ranging from 1 to 1.36 eV. The new configuration of the deep traps can be originated by defect reactions involving association and/or dissociation of complex defects, when the electronic occupation is changed by these photons. In this context, it should be very interesting to test the behaviour of shallow traps during the defect reaction mechanism. Therefore the photocurrent is studied in the spectral region of the band gap edge, after excitation with 1 to 1.35 eV photons. The results obtained confirm that shallow trap concentrations are strongly influenced by variations in the charge state of deep traps. Thus samples characterized by optical quenching of the extrinsic photocurrent undergo a complete quenching of the near intrinsic photocurrent (1.45 eV band), whereas samples showing optical enhancement of the extrinsic photocurrent, exhibit an enhancement of the 1.45 eV near intrinsic photocurrent band, whose maximum is also shifted towards 1.475 eV.

I1 est bien connu que les photons d’energie comprise entre f et 1,36eV produisent de grosses transformations sur le photocourant du GaAs massif semi-isolant. I1 existe deux effets photo- memoire qui sont fortement dependants de la sorte d’6chantillon: il y a les bchantillons qui pr6sen- tent une augmentation du photocourant extrinshque dans la region (1 i 1,35 eV) aprbs une longue excitation B T = 120 K, I’autre varietB prksente un quenching optique du photocourant dans les mdmes conditions d’excitation. Ces phbnomhnes peuvent dtre expliquBs sur la base de changements importants dans la configuration de certain8 pibges profonds sous excitation comprise entre 1 et 1,35 eV. La nouvelle configuration des pibges profonds peut avoir pour origine des reactions de dbfauts provenant de l’association ou dissociation de dBfauts complexes oh I’occupation Blectronique est changee par ces photons. Dans ce contexte, il est interessant de regarder le comportement des pihges peu profonds durant ce processus. Pour cela, nous avons Btudie le photocourant dans la region du gap aprhs excitation B 1 B 1,35 eV. Les rksultats obtenus confirment que les concentrations de pieges peu profonds sont fortement influencees par les variations de 1’6tat de charge des pihges profonds. Les Bchantillons caraotBris6s par le quenching optique du photocourant extrinshque subissent un quenching complet du photocourant intrinsbque (1,45 eV); en contre partie les Bchantillons montrant un accroissement du photocourant extrinshque, presentent un accroissement de la bande centree B 1,45 eV, le maximum de cette bande glissant jusqu’il 1,475 eV.

l) 47011 Valladolid, Spain. 2, CEM Universite des Sciences et Techniques du Languedoc, 34060 Montpellier, France.

624 J. JIMENEZ, J. BONNAFE, P. HERNANDEZ, and J. A. DE SAJA

1. Introduction

There is a great interest in the use of semiinsulating GaAs as a substrate for IC technology. In recent years a great deal of attention has been paid to the study of midgap levels which control the properties of the substrate and therefore of the de- vices.

It is usually assumed that the most important deep donor in GaAs is the so-called EL2 level in Martin's notation [l]. I n spite of the many attempts that have been made in the last few years, its physico-chemical nature still remains unknown. Recently, it has been claimed that the origin of this centre is the As@, antisite [2, 31, which is created during the post-growth cooling of GaAs ingots. However, this assertion does not meet with everybody's approval and there is a tendency to relate EL2 to a complex defect built-up around the antisite [a]. This centre has very unusual properties, the most important of which is the photoquenching of their photosensitivity when the sample is submitted to a persistent optical excitation with photons of energy higher than 1 eV [5 to 81. This behaviour has been claimed to be due to a strong electron-lattice coupling of EL2 which is transformed into a metastable state [6, 91.

I n some recent papers we demonstrated that there is another photomemory effect that can be observed in some GaAs samples [lo to 121. However, this effect which is generated by the same photons used in the photoquenching exhibits an entirely different type of behaviour. In fact, the photoresponse of these samples undergoes an enhancement instead of the expected quenching.

Our purpose in this paper is to study the influence exerted on the photoresponse of the near-intrinsic spectral region by transformation of the samples into those sensitivity states.

2. Experimental Method

The sample used for measurements were high-resistivity bulk GaAs, cut in the (100) plane from HB (horizontal Bridgman) ingots. Typical dimensions of the samples were 5 x 2 x 0.5 mms. The samples were chemically etched in a H,S04:H,0,:H,0 (50: 15: 15) solution, cleaned in acetone and mechanically polished.

Photocurrent measurements were carried out either with a log picoammeter (Keithley 26000) or a digital electrometer (Keithley 610). The optical excitation was produced by ligth from a tungsten lamp, passing through a Bausch - Lomb grating monochromator and a filter system. The photocurrent under these excitation conditions was always significantly higher than the dark equilibrium current. Electric con- tacts were made with silver paste annealed a t 300 "C or alloying with indium.

3. Results and Discussion

The study of the photocurrent in several samples of semiinsulating GaAs revealed two different extrinsic photomeniory effect. I n other previous papers we have studied these effects [7, 10 t o 121. Basically these phenomena can be described as follows:

3.1 Photoquenching

This effect is typical of GaAs; it has been observed by several experimental methods [5, 6 t o 8, 131. Their existence has been claimed to be the best signature of the EL2 level [14]. It is produced by excittation with photons ranging from 1 to 1.25 eV in the low-temperature domain (T < 120 K). Under these conditions the sample is driven into a low-sensitivity state, which remains a s long as the temperature is kept below

Optically Induced Photomemory by 1 to 1.35 eV Photons on Bulk GaAs 625

1 2

re I I ’ ” I ’ I I I I I I I - -

----_____ 5-

I0-N f 1 f I

Fig. 1. Time evolution of 1.13 eV extrinsic photocurrent for two different samples: (1) sample showing photoquenching, (2) sample characterized by photogeneration

+ (T = 77 K)

140 K. In Fig. 1, we show the evolution of the 1.13 eV extrinsic photoconductivity as a function of time; the photoquenching is clea.rly observed after several minutes of excitation.

3.2 Photogeneration This effect has been observed by us in many semiinsulating GaAs samples in photocurrent experiments [lo to 121. After sufficiently long excitation with photons of energies ranging from 1 to 1.35 eV the sample is transformed to a high-photosensitivity state, which remains as long as the temperature is below 125 K. Above this temperature, the photocurrent associated to this photosensitivity state is thermally quenched and the sample is then driven to its normal photosensitivity state. We have termed the high-photosensitivity state “on-state”. In Fig. 1, we show the formation of the

on-state” as a function of excitation time. The time needed to built up the “on- state” is both a function of the photon flux and of the energy of photons. 6 6

3.8 Discussion

There are controversies about the nature of the midgap levels in GaAs after excitation. In our description of these photomemory effect,s, we have developed phenom- enological models dealing with defect reactions. These defect reactions should be induced by changes in the charge state of the deep defects during the optical excitation. These models should explain the changes of the photosensitivity of samples by the formation of new complex defects or by the dissociation of existing complex defects. A model of this kind has been developed by Levinson to explain the annealing of defect,s created by electron irradiation in InP [15, 161 and to explain the properties of EL2 in GaAs [17].

It is interesting to note that in a model of this type, the role played by shallow levels must be important. Thus, it is well known that some photomemory effects of GaAs after excitation with energies higher than 1 eV have been reported in the near-intrinsic spectral region. Hence photoquenching of the near-intrinsic photocurrent [7, 18, 191, quenching and enhancement of the excitonic photocurrent [20], enhancement of the TAV (transverse acoustic voltage) in the same spectral region [21], quenching of the optical absorption in the band edge region [22], and strong changes in the TSC (thermally stimulated current) of shallow traps [23, 241 occur. All these results suggest that strong transformations are induced in the shallow trap distribution after sufficiently strong optical excitation of midgap levels.

We have studied the evolution of the near-intrinsic photocurrent as a function of the time excitation with 1.13 eV photons for both kinds of samples. Thus in Fig. 2

I I I I I I

J. JIMENEZ, J. BONNAFE, P. HERNANDEZ, and J. A. DE SAJA

Fig. 2. Sample undergoing extrinsic photoquenching. The near-intrinsic photocurrent spectrum is reported after different excitation periods, t , with photons of 1.13 eV: (1) t = 0, (2) 5, (3) 10, (4) 25 min (T = 77 K)

Fig. 3. Sample undergoing extrinsic photogeneration. The near-intrinsic photocurrent spectrum is reported after different excitation periods, t , with photons of 1.13 eV: (1) t = 0, (2) 2, (3) 4, (4) 6, (5) 10, (6) 25 min (T = 77 K)

we show this evolution for samples having photoquenching. The near-intrinsic photocurrent spectrum shows a peak at 1.45eV and a shoulder at 1.47eV. This peak presumably corresponds to A-D type transitions. After the excitation with 1.13 eV photons the near-intrinsic photocurrent is gradually quenched ; finally a full quenching is attained.

The samples showing photogeneration exhibit a photocurrent peak at 1.46 and a shoulder at 1.485 eV (Fig. 3). Its evolution, after excitation with 1.13 eV is shown in Fig. 3. An enhancement of the photocurrent peak has been observed in these samples, the peak maximum has also shifted towards 1.475 eV. The photocurrent is also enhanced for energies lower than 1.44 eV. This last enhancement seems to be the tail of the extrinsic photogeneration which extends to more than 1.4eV. This result agrees with the TAV measurement of Davari and Das [21].

These results show that the shallow trap distribution is strongly affected by optically induced transformations in deep traps. In Fig. 4 we show the time evolution of both the extrinsic and the near-intrinsic quenching, it is seen that the former is faster than the latter.

The same has been done for the photogenerated samples. The formation and enhancement of the 1.475 eV band is slightly slower than the photogeneration of the ( 6 on-state” in the extrinsic spectral region.

Optically Induced Photomemory by 1 to 1.35 eV Photons on Bulk GaAs 627

Fig. 4. Iph(i?)/Iph(O) vs. time excitation with 1.13 eV photons: (1) extrinsic photocurrent (1.13 eV), (2) near-intrinsic photocurrent (1.45 eV) (T = 77 K). It is clearly observed that the near- intrinsic quenching is done only after the extrinsic quenching was achieved

These results provide an interesting insight into the nature of the reaction defect model. Thus, we can say that the first step of the process is the relaxation of the deep level when it is ionized by the extrinsic light.

The photoquenching could be seen as follows: the 1.13 eV light ionizes EL2 giving an EL2* level plus a free electron in the L conduction band. There is then a com- peting process between the capture of the electron by EL2 without atomic rearrangement at the defect and the capture with atomic rearrangement leading the metastable EL2*. This last mechanism can be considered as a case of REDR (recombination- enhanced defect reaction) [25]. This first stage of the reaction corresponds to the quenching of the extrinsic photocurrent.

Once the metastable state EL2* is reached, the reaction can continue under extrinsic excitation by electrostatically capturing shallow donors and/or acceptors (<0.07eV from the top of the corresponding band), thus forming a new complex for which no near-intrinsic transitions take place.

The photogeneration acts in a similar way. In fact, in this case, the optical excitation creates photosensitive traps by means of a defect reaction mechanism acting in a similar way. The new metastable state which is formed, is different from EL2* because it is photosensitive. In previous papers, we have compared this trap with the EL6 due to thermal emission similarities we have found; i.e. we have obtained a thermal acti- vation energy of 0.31 eV from TSC, which corresponds to that usually reported [26, 271. As it occurs for the photoquenching, the new metastable state for EL6 can intro- duce changes in the shallow trap distribution by electrostatic interaction under extrinsic excitation. These changes are reflected in the near-intrinsic photocurrent spectrum which is enhanced and shifted (Fig. 3). It is important to note that this process is accomplished in the presence of free carriers in contrast to what occurs for the photoquenching where the extrinsic photocarriers do not exist. Therefore, in the transformation of the shallow trap concentration by the “on-state”, screening by free carriers must also be considered. The fact that the photocurrent band shifts towards 1.475 illustrates the change in the ND-Na4 concentration after the defect reaction has been accomplished.

All these data confirm that the midgap levels of GaAs are associated to complex defects rather than to point defects. Moreover, the fact that both photomemory effects, photoquenching and photogeneration, were created by the same photons, suggests that both complex defects EL2 and EL6 could have some common compounds. Furthermore, the thermal annealing of both metastable states is achieved in the same temperature range which should support the above assertion.

In conclusion, we have shown that the shallow donors and acceptors (0.07 eV from the top of bands) are able to react with the midgap defects, when these are transformed by means of the optical excitation with photons of the 1 to 1.35 eV spectral range.

628 J. JIMENEZ et al. : Optically Induced Photomemory on Semiinsulating GaAs

Acknowledgements

The authors are indebted to Prof. J. P. Fillerd (CEM, Mont,pellier) for fruitful dis- cussions and to Mr. c. Poiblaud (RTC (Caen) for supplying the samples used in this work.

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Optically induced photomemory by 1 to 1.35 eV photons in the near-intrinsic spectral region on semiinsulating bulk GaAs