Migration of silicon atoms in planar-doped GaAsAlGaAs modulation doped fluid effect transistor heterostructures grown by molecular beam epitaxy

MAllmlALS S@IEI~E & ENGINEERING ELSEVIER

Materials Science and Engineering B35 (1995) 149-155

B

Migration of silicon atoms in planar-doped GaAs/A1GaAs modulation doped fluid ,effect transistor heterostructures grown by molecular beam epitaxy A.T.G. Carvalho 1, A.G. de Oliveira, E.S. Alves, M.V. Baeta Moreira Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, Caixa Postal 702, 30161-970 Belo Horizonte, Minas Gerais, Brazil

Abstract

Photo-Hall and Shubnikov-de Hass measurements were performed on four silicon planar-doped A10.3Ga0.7As/GaAs MODFET heterostructures. We varied the nominal silicon concentration (4.0 x 10~2 and 6.2 x 1012cm-2), the growth temperature (500 °C and 620 °C) and the distance between the planar-doped layer and the heterojunction (40 ~ and 100 A). We obtained the carrier mobility against carrier density curves by changing the carrier density using the persistent photoconductivity effect. An IR photo-diode was used to illuminate the samples. Parallel conduction in two channels, the heterojunction channel and the planar-doped layer, was observed for three samples. It was not observed for the sample grown at 500 °C with the 100/~ spacer. The effects were explained taking account of the silicon profile. Our results also indicate that, for the samples grown at 500 °C, the interface roughness is the dominant scattering mechanism limiting the mobility of the two-dimensional electron gas formed at the heterojunction, while ionized impurities are dominant for samples grown at 620 °C.

Keywords: Heterostructures; Molecular beam epitaxy

1. Introduction

Modulation-doped field effect transistor ( M O D F E T ) heterostructures are of fundamental importance in modern high-speed microelectronics. The essence of modulation-doped heterostructures of A1GaAs/GaAs is the spatial separation of the ionized dopant atoms, which are located in the larger band gap material (A1GaAs), from the electrons which are transferred to the smaller band gap and undoped material (GaAs). This is due to the fact that the donor level in the AIGaAs layer has a higher energy than that in the G a A s layer. This electron transfer causes substantial band bending resulting in the formation of a quasi triangular potential well in the interface. The electrons will be confined in this well owing to the electrostatic interaction with ionized dopant atoms, and they will behave as a quasi two-dimensional electron gas 1 On leave from Departamenlo de Fisica, Universidade Federal de Vi~osa, 36570-000-Viqosa, Minas Gerais, Brazil. 0921-5107/95/$09.50 © 1995 - - Elsevier Science S.A. All rights reserved

S S D I 0921-5107(95)01332-6

(2DEG). Consequently in such structures, the scattering rate due to the ionized dopant atoms is reduced, leading to a dramatic enhancement of the electron mobility, especially at low temperature. Another peculiarity of n-type uniformly doped alloy in the AlxGa] xAs system is the presence of a persistent photoconductivity effect (PPC) arising under illumination at T ~< 120 K for an aluminum content higher than about x = 0.22. It is now well accepted that the PPC effect arises from the silicon atoms at the crystallographic site of the group III element, where they form beyond typical shallow donors, another deep level called the D X center [1]. Photoexcited electrons from the D X centers remain free for a long time because the probability of electron recapture by the ionized donors is reduced drastically owing to a potential barrier. In the presence of a heterojunction, the free electrons generated in the doped A1GaAs layer, from the shallow donors and from the D X center, move into the interface channel. Incomplete transfer may leave residual electrons in the embedded planar region and allow,

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A.T.G. Carvalho et al. / Materials Science and Engineering B35 (1995) 149 155

therefore, the possibility of parallel conduction [2]. The presence of parallel conduction might reduce drastically the performance of M O D F E T s [3]. Advances in epitaxial growth techniques have allowed enormous flexibility in the fabrication of heterojunctions. In particular, the doping impurities can be confined within a single atomic plane leading to the concept of planar doping [4]. In a real planar-doped structure, however, the dopant atoms spread outside the single plane, because diffusion and surface segregation effects contribute to enlarge the silicon profile. The results already published [4-8] show that the ideal "V"-shaped potential well of a real doped plane would be altered self-consistently if the silicon distribution broadened because of redistribution of the silicon atoms during growth. The idea of replacing the uniformly doped AIGaAs layer by a planar-doped layer came about as a tentative solution to two problems. The first point was related to the expectation of increasing the charge transfer efficiency across the heterojunction interface into the quantum well, and therefore increasing the carrier density compared with conventional modulation-doped structures [4,9]. The second point concerns the DX center. At present, very little is known about the DX center in planar-doped structures. Indeed, even the existence of the DX center in two-dimensional structures is still an open problem. Assuming the absence, or at least low efficiency, of the DX center in the planardoped structure, the deleterious effect of the DX center should be eliminated or at least somewhat reduced [10]. The negative aspect of this replacement concerns parallel conduction, since the planar-doped layer tends to favor the formation of the second channel associated with the depth of the "V"-shaped potential. The band profile of the M O D F E T structure is determined by the active doping concentration [11,12]. This in turn is determined by the effective silicon profile and by the distance between the AIGaAs GaAs interface and the planar-doped layer (the so-called spacer in the present work), which are therefore essential growth parameters that should be well controlled in order to determine desirable transport properties. The morphological quality of the A1GaAs-GaAs interface is another growth parameter which is decisive in determining the electrical properties of the samples. In the present work, we investigated the transport properties of planar-doped A1GaAs/GaAs heterostructures grown by MBE. Basically we vary the planar-doped nominal silicon concentration, the growth temperature and the spacer. Our approach consists in obtaining the electrical parameters, at different temperatures, by varying the carrier concentration using a light source. Therefore, free carriers will be produced by photoexcitation and because of persistence effects the new transport properties might be preserved even after illumination. The

occupancy of higher subbands in the interface well, using magnetoresistance measurements (Schubnikovde Hass oscillations), was observed to occur for all samples. We show some evidence that parallel conduction will be present for some samples at our limit of high intensity illumination, associated with the formation of a second spatially separated channel in the planar-doped region.

2. Experimental details The four M O D F E T structures used in the present study were grown by MBE. Typical structures begin with a 0.7 lam undoped GaAs buffer layer grown on a semi-insulating Cr-doped GaAs (100) substrate. The GaAs buffer layer was followed by an undoped A10.3Ga0.TAS spacer layer, with a thickness of 40/k or 100/~. After the spacer, the growth was interrupted by closing the Ga and A1 shutters, and silicon atoms were deposited to achieve the desirable nominal silicon concentration Nsi. The planar-doped layer was then buried by a 400/k undoped A10.3Gao.TAS layer and the structure was finished with a 50/k thick GaAs cap layer. The GaAs cap layer was heavily doped with silicon atoms (5.0 x 1018cm-3). Some of the samples and growth parameters are listed in Table 1. The magnetotransport parameters of the samples were determined using a standard Hall bridge defined photolitographically. Contacts were made by allowing indium into the layers at 400 °C for 10 min. Finally, the samples were mounted on a header and bonding wires were attached. The Hall measurements were performed in a heliumgas cryostat. A fully automated system enabled measurements to be taken over the temperature range from 10 K to room temperature. To determine the Hall free electron concentration, Vxy was measured both in the presence of a magnetic field B (B = 1 T) and in the absence of it. The switching time between the " o n " and "off" conditions for the magnetic field was 30 s, i.e. we could only resolve transients that were larger than this time scale. For the Shubnikov-de Hass (SdH) measurements, carried out at 4.2 K, the samples were inserted in an Oxford cryogenic dewar equipped with a variable temperature control and a superconducting magnet Table 1 Principal characteristics of samples Sample

TS (°C)

Spacer (/~)

Nsi (x 1012cm-2)

1 2 3 4

500 500 620 620

40 100 40 100

6.2 6.2 4.0 4.0

A.T.G. Carvalho et al. / Materials Science and Engineering B35 (1995) 149-155

providing a transverse magnetic field B up to 15 T. The measurements were carried out with a d.c. current of 20 laA applied tc, the sample, and the carrier concentration in the samples was varied by illuminating the samples with an IR L E D (with hv centered at 1.4 eV at low temperatures). For the Hall experiments carried out at 77 K and 20 K, the samples were illuminated continually starting with /LED = 10 ~tA and increasing the current ~Lo 30mA, where saturation conditions were expected. For measurements carried out at 4.2 K, the sample,; were illuminated by flashing the LED, in controlled ]ight doses, and between two successive light doses the magnetic field was varied from zero to 15T to obtain the SdH traces. The recovery times for the persistent effect were large enough to assure that both methods, continuous illumination and flashing, gave almost the same result at 4.2 K. In this way, we could trace typical SdH oscillations for different carrier densities up to the saturation condition, defined for this work as the maximum free carrier density obtained under the highest light intensity of the LED. The individual subband occupancies were determined using standard fast Fourier transform (FFT) methods.

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3. Results Fig. 1 shows the Ft vs. n dependence for the four samples, measured at 77 K. To generate these curves a large number of individual measurements, around 500, were carried out, so that small details were expected to be observed. The samples with the 100 A spacer, i.e. samples 2 and 4, show a sharp rise in mobility. Just at the sta:rt of the illumination, with a very faint illumination intensity, the two curves present a kink, characterizing a change in slope of the curve. After the kink /~ increases smoothly with n. The samples with the 413 A spacer, i.e. samples 1 and 3, show in contrast a slight initial rise in mobility which is then followed by a large drop in mobility. In this case, the final values of the mobility are significantly smaller than the initial values. Sample 3 presents a local minimum for n ~ 1012 cm -2. Fig. 2 shows, preserving the same order of presentation as Fig. 1, the /~ vs. n dependence measured at 4.2 K and the SdH oscillations for four points of the /~ vs. n curve. For each sample, the four SdH oscillations presented are identified by letters a to d, for increasing values of n. Ill this case however, owing to limitations of the experimental procedures, only a small number of measurements (on the order of ten) were carried out. In spite of the small number of measurements the main trends for the /~ vs. n curves observed at 77 K and 20 K are preserved. It should be noted that, on reducing the temperature from 77

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i

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Fig. 1. The measured mobility/t as a function of Hall concentration n induced by illumination at 77 K for samples (a) I and 2, (b) 3 and 4.

to 4.2 K, the mobility increases significantly for the samples grown at 620 °C (Fig. 2(b)), which is consistent with the reduction in phonon scattering efficiency. For the samples grown at 500 °C (Fig. 2(a)), however, the improvement in mobility is not significant so phonon scattering is not the determining factor for the mobility. The SdH oscillation traces observed in darkness for all four samples are easy to interpret because they present simple forms. However, the SdH oscillations observed under saturation conditions, i.e. those points having the maximum values of carrier concentration, are more difficult to interpret because they present complex oscillation traces. The magnetoresistance for samples 3 and 4 shows clearly SdH oscillations superposed on a rising background and the minimal SdH oscillation does not reach zero. For sample 1 the SdH oscillations do not reach zero but the background is reasonably constant. In this case however there is a clear modulation in the oscillation trace.

152

A.T.G. Carvalho et al. / Materials Science and Engineering B35 (1995) 149-155

4. Interpretation

and discussion

4.1. Parallel conduction In Table 2, from the frequency of the SdH oscillations and using standard fast Fourier transform methods, we list the subband occupations (hi values). The results show that, in darkness, only one subband is occupied (no) and we identified it with the subband of the 2DEG channel, because the mobilities have values typical of modulation doped structures having a small spacer. This result confirms the calculations [11], which show that most of the channel electrons are in the lowest subband. In the case of only one subband occupation the Hall measurements nH are expected to be a good assessment for the real free carrier concentration of the first subband, because in a single channel the values obtained using both techniques might agree. Indeed, this was exactly what was observed, i.e. for all samples in darkness, no--~nn. The presence of small kinks, in the/~ vs. n curves for

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samples 2 and 4, shown in Fig. 1, has been associated usually with the onset of the second subband occupation for the 2DEG channel [13,14]. The presence of the kink has been explained on account of the fact that when the Fermi energy rises above the second subband it starts to be populated and a new scattering process begins (intersubband scattering). Occupation of the second subband therefore produces changes in the mobility. Different subbands present different mobilities, so changes are expected in the Hall mobilities when the second subband is populated, producing the observed kink. For a further increase in the carrier density, the mobility recovers its tendency to show a smooth and monotonic curve. Our results corroborate this assumption because the SdH data indicate clearly the occupation of two subbands just after the appearance of the kink. Eventually, other higher subbands are expected to be populated. However, this is not observed for our samples from the SdH oscillation traces. Changes in the mobility, in this case identified as a maximum just at the start of illumination, are also observed for samples 1 and 3. The occupation of the second subband in the 2DEG channel is also confirmed by the SdH results. The main difference for these two samples, however, occurs on further illumination. In contrast with samples 2 and 4, the y values reduce on further increasing the carrier concentration. This reduction is much stronger for sample 3 than for sample 1. We believe that parallel conduction between the 2DEG channel and another spatially separated channel having carriers with low mobilities is responsible for the observed differences. It should be noted that the observed mobility values for sample 3, which was grown at 620 °C, are much higher than those observed for sample 1. The presence of the SdH oscillations superimposed on a rising background and the fact that the minimum of the SdH oscillations does not go to zero, observed for samples 3 and 4 (d in Fig. 2(b)), are considered to be the signature of parallel conduction in spatially separated channels [15]. Although sample 1 does not present a clear rising background, the fact that it does not go to zero and the presence of a third SdH frequency suggest the existence of parallel conduction in this sample also. The SdH data also indicate, using the fast Fourier transform method, the population of a third channel for these three samples, which is also consistent with the assumption of parallel conduction. This third channel indicates the presence of a carrier density of the order of no. On applying the model of parallel conduction in two separate channels [16] a low-mobility carrier is deduced to account for the strong drop in mobility observed in the y vs. n curves plotted in fig. 1. This suggests the presence of second channel at the planar-doped layer. It is well known that typical planar-doped channels present carrier mobilities

153

A . T . G . C a r v a l h o et al. / M a t e r i a l s S c i e n c e a n d E n g i n e e r i n g B 3 5 ( 1 9 9 5 ) 1 4 9 - 1 5 5

Table 2 Subband occupation n~, mobility/~n and Hall carrier concentration nH for different controlled light doses Sample insert

gH ( x 10a crn2 V - l s -1)

nH (1011 cm -2)

no

(1011 cm -2)

(10 II Cln-2)

11.3 11.5 12.4 18.6

11.3 11.2 11.5 17.3

-0,81 1.11 2.14

0.18 0.19 0.20 0.31

.

n1

A = (n o + n l ) / N s i

la lb lc ld

1.73 1.68 1.78 0.32

2a 2b 2c 2d

2.06 4.22 6.15 12.1

3.73 4.82 6.88 8.90

3.74 5.02 6.59 8.32

--0.65 0.89

0.06 0.08 0.12 0.15

3a 3b 3c 3d

10.9 10.1 10.9 7.61

8.31 l 1.1 12.1 15.6

8.29 10.1 11.2 13.7

-0.79 1.07 1.66

0.21 0.27 0.31 0.38

4a 4b 4c 4d

6.92 10.1 15.4 18.4

4.95 6.28 7.05 7.79

5.13 6.02 6.83 7.26

-0.19 0.28 0.48

0.13 0.16 0.18 0.19

two to three orders of magnitude lower than the 2DEG channel. The existence of this second conduction channel has been proved for both conventional and planardoped MODFET structures [17-19]. The charge transfer efficiency from the donor impurities to the 2DEG channel may depend on a number of factors, such as the cortduction-band offset, nominal silicon doping, the width of the embedded region for a uniformly doped layer, and its position relative to the heterointerface [2,11,12]. The same factors are expected to be relevant for the establishment of parallel conduction. Our samples have tlhe same aluminum content, so that band offset is not a factor to be considered in the present work. The appearance of parallel conduction is strongly dependent on the spacer width. For narrow spacers the characteristic of the spatial separation of the two channels tends to disappear, tending to a single broad but distorted channel [20]. Diffusion, occurring during growth, will spread the silicon profile so that the nominal spacer should be replaced by an effective spacer, both for theoretical c,alculation and experimental fitting. On the AIGaAs side, the presence of a high concentration of ionized silicon atoms will produce an efficient scattering mechanism, and carriers having low mobilities are expected. Because of this, low mobility levels will be dominant, if the wave functions for electrons have amplitudes predominantly on the AIGaAs side. Conversely, high mobility levels are expected for wave functions predominantly on the GaAs side. The existence of levels having relatively high mobilities on the GaAs side will be tile dominant factor in defining the Hall mobility. For t]~e regime of large spacers, the planar-doped channel will be dominant, because the

probability of transferring electrons into the heterojunction will be drastically reduced. In this limit, the carrier density in the high mobility 2DEG channel is so low that its contribution to charge transport is negligible. In spite of the fact that the mobilities are drastically smaller in planar-doped channels than those observed in 2DEG channels, the transport properties are defined by the low mobility channel owing to the high carrier density. The second important parameter in defining the presence of parallel conduction is the amount of silicon atoms used in doping the AIGaAs layer, using either homogeneous or planar doping. Once the spacer is fixed, more electrons are transferred to the 2DEG channel with an increase in the nominal silicon atoms in the planar-doped layer. The Fermi level will increase with the carrier density in the 2DEG channel. Ionized silicon atoms left behind will establish the planar-doped well, and a threshold concentration is necessary to populate the planar-doped channel, i.e. to form a well deep enough to reach the Fermi level. This point is being studied at the moment for a larger number of samples. Only sample 2 does not present the SdH characteristics of parallel conduction. We believe that the free carrier concentration is the key parameter to explain this fact. This sample presents the smallest values of carrier concentration. Even though the nominal silicon concentration is higher than that of sample 4, sample 2 presents a lower carrier density. This can be explained based on the growth temperature. Diffusion is almost negligible for growth temperatures around 500 °C, but is not at 620 °C, so that the effective spacer is smaller for sample 4 and more electrons are expected to be

A.T.G. Carvalho et al. / Materials Science and Engineering B35 (1995) 149-155

154

transferred for this sample, as in fact was observed. In spite of the fact that the average electron distance from the interface increases for decreasing channel carrier density, our result suggests that diffusion is the dominant effect. An important parameter determining the transfer of electrons from donor impurities in the planar-doped layer to the 2DEG channel is the position of the Fermi level relative to the bottom of the GaAs conduction band at the interface. The Fermi level increases with the channel carrier density. According to calculations [11], the Fermi level and the difference between the fundamental and the first excited subband, at 0 K, increases around 100%0 and 40% respectively, when the carrier density in the 2DEG channel increases from 1 × 10 ~ cm -2 to 5 × 10 z~ cm -2. This result suggests that the low carrier density observed for sample 2 maintains the Fermi level below the first subband in the planar-doped channel, even at saturation conditions. In this condition, therefore, the threshold for parallel conduction was not reached. In Table 2 we also list the ratio A between (no + n~) and the nominal silicon concentration Nsi. As expected, for samples grown at the same temperature, the A values in the dark for samples having the 40 A spacer are higher, at least double, than for the sample having the 100 A spacer. Comparing samples with the same spacer, one can see that more charge is transferred for samples grown at 620 °C. This corroborates the assumption that diffusion contributes to reduce the spacer, i.e. the effective spacer for samples grown at 620 °C is smaller than the nominal spacer. The silicon profile becomes broad for increasing growth temperatures [4] owing to diffusion.

4.2. Effect of the surface quality on the mobility Because the samples were grown at two different temperatures (500 °C and 620 °C) it is possible to correlate the effect of this growth parameter with interface quality and transport properties. We also analyze, the spread in the silicon profile considering the transfer efficiency from the doped layer into the 2DEG channel. Transport measurements can be used to determine two characteristic relaxation times, the transport lifetime rt and the quantum lifetime 17q [21]. The transport and quantum lifetimes are not identical owing to the presence of an extra (1 - cos 0) factor in the expression for rt, i.e. 1/~ t =

fdk'P(k, k')(1 - cos 0)

1/Zq = f d k ' P ( k ,

k')

(1) (2)

Here, P(k, k') is the scattering probability from state k to state k', and 0 is the scattering angle. Therefore, zt is

insensitive to small-angle scattering while rq is sensitive to all scattering events. The transport lifetime, defined by solution of the Boltzmann equation in the relaxation time approximation, is related to the Hall mobility through PH = e~t/m*, where m* is the electron effective mass. Experimentally, the quantum lifetime Zqi for individual subbands can be determined through analysis of the Fourier transforms of the SdH oscillation peaks [22]. The width of a Fourier peak is inversely proportional to the mobility (flqi "= eTqi/m*) and the peak amplitude should be proportional to the square of the mobility. The amplitude of magnetoresistivity Pxx oscillations to the first harmonic is given by the following expression:

Pxx = ~o + ~ ~ri exp( -

7c/piB )

cos(hni/2eB)

(3)

The half-width at half-height of the Fourier power spectrum of the oscillations, for the first derivative of resistivity, is given by ~(41/3-

1)

6ui =

(4)

The first subband quantum mobilities /tqO estimated using Eq. (4) for all samples are listed in Table 3. We list also the ratio between "rt/'[q0 = Pt/Pq0, where for Pt the pi~ values listed in Table 2 were used. In systems where the scattering process is via mostly small angles, which occurs for example for impurity scattering, the transport rt to quantum rq lifetime ratio can be significantly higher than unity. Conversely, in systems with isotropic scattering, which occurs for example for interface roughness scattering, we usually have ~'t/rq ~ 1 [21-23]. One can see from the results listed in Table 3 that the rt/z" q ratio, in darkness, is about unity for the two samples grown at 500 °C, suggesting that the main scattering process for these samples is associated with the surface roughness. Indeed, it is well known that samples grown at low temperature produce bad quality GaAs-AIGaAs interfaces. This fact is also consistent with the observation that the mobility values, in darkness, do not change with temperature, i.e. phonon scattering is not dominant in determining the mobiliTable 3 First subband quantum mobilities/-/qo estimated and the ratio Sample

,ttqi (xl04cm2V -Is -l)

-tt/l0

1

1.70

1.02

2 3 4

1.60 2.44 1.98

1.29 4.55 3.49

rt/~'q0

A.T.G. Carvalho et al. / Materials Science and Engineering B35 (1995) 149-155

ties. Conversely, for the two s a m p l e s g r o w n at 620 °C the zt/z q r a t i o is m u c h higher t h a n unity, suggesting t h a t the m a i n m e c h a n i s m limiting the carrier mobilities is ionized impurities.

5. Conclusions Parallel c o n d u c t i o n is an effect t h a t m u s t be considered in M O D F E T heterostructures. The a p p e a r a n c e o f this effect reduces the device p e r f o r m a n c e . W e have shown t h a t this effect has d r a m a t i c consequences in p l a n a r - d o p e d M O D F E T heterostructures. O u r results indicate t h a t the second channel occurs in a s s o c i a t i o n with the m i g r a t i o n o f silicon a t o m s o f the p l a n a r - d o p e d layer. This seems to be a d i s a d v a n t a g e o f p l a n a r - d o p e d structures because paralle,1 c o n d u c t i o n occurs m o r e easily t h a n for b u l k d o p e d structures. W e used the persistent p h o t o c o n d u c t i v i t y effect to induce the a p p e a r a n c e o f parallel c o n d u c t i o n . T y p i c a l l y we observe t h a t there is a c a r r i e r density t h r e s h o l d for the r e d u c t i o n in the effective m o b i l i t y o f the device a n d it was a s s o c i a t e d with a p p e a r a n c e o f the s e c o n d channel. This was identified by S h u b n i k o v - d e H a s s oscillations. The results s h o w t o o t h a t in s a m p l e s g r o w n at 500 °C, the interface r o u g h n e s s is the d o m i n a n t scattering m e c h a n i s m which limits the m o b i l i t y o f the', t w o - d i m e n s i o n a l electron gas f o r m e d at the interface, while in s a m p l e s g r o w n at 620 °C ionized i m p u r i t i e s are d o m i n a n t .

Acknowledgments T h e a u t h o r s w o u l d lJike to t h a n k F I N E P , C N P q , C A P E S a n d F A P E M I G for financial s u p p o r t ,

155

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