Space-charge-limited currents in GaN Schottky diodes

Solid-State Electronics 49 (2005) 847–852 www.elsevier.com/locate/sse

Space-charge-limited currents in GaN Schottky diodes X.M. Shen

a,*

, D.G. Zhao b, Z.S. Liu b, Z.F. Hu a, H. Yang b, J.W. Liang

a,b

a

b

Institute of Semiconductor and Information Technology, Tongji University, 1239 Siping Rd, Shanghai 200092, PR China State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China Received 19 June 2004; received in revised form 2 February 2005; accepted 12 February 2005

The review of this paper was arranged by Prof. A. Zaslavsky

Abstract Unusual dark current–voltage (I–V) characteristics were observed in GaN Schottky diodes. I–V characteristics of the GaN Schottky diodes were measured down to the magnitude of 10
14 A. Although these Schottky diodes were clearly rectifying, their I–V characteristics were non-ideal which can be judged from the non-linearity in the semi-logarithmic plots. Careful analysis of the forward bias I–V characteristics on log–log scale indicates space-charge-limited current (SCLC) conduction dominates the current transport in these GaN Schottky diodes. The concentration of the deep trapping centers was estimated to be higher than 1015 cm
3. In the deep level transient spectra (DLTS) measurements for the GaN Schottky diodes, deep defect levels around 0.20 eV below the bottom of the conduction band were identified, which may act as the trapping centers. The concentration of the deep centers obtained from the DLTS data is about 5 · 1015 cm
3. SCLC measurements may be used to probe the properties of deep levels in wide bandgap GaN–AlGaN compound semiconductors, as is the case with insulators in the presence of trapping centers.
2005 Elsevier Ltd. All rights reserved. PACS: 71.55.Eq; 73.20.Hb; 73.40.Ei Keywords: GaN; Schottky diode; I–V characteristics; Space-charge-limited current

1. Introduction Despite the rapid progress in materials growth, the crystal perfection of III-nitrides still remains greatly inferior to conventional III–V semiconductors. Consequently, considerable concentrations of various deep levels are contained inevitably in GaN based materials [1]. Due to their wide band gaps, effects of deep level centers on the III-nitride materials and devices could be more pronounced than in narrower band gap semi*

Corresponding author. Tel.: +86 216 598 2310; fax: +86 216 598 4898. E-mail address: [email protected] (X.M. Shen). 0038-1101/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2005.02.003

conductors. In fact, deep level associated effects such as persistent photoconductivity (PPC), current collapse, etc., have been observed in a wide variety of III-nitride materials and structures [2,3]. The presence of these effects indicates possible carrier trapping effects in IIInitride devices, which could cause instabilities in such devices and hence have significant influences on the device performance. Recently, Hall et al. [4] reported on deep-level dominated rectifying contacts for n-type GaN films. They found that the deviation of the I–V characteristics of the rectifying contacts from thermionic emission theory could be interpreted in terms of spacecharge-limited current (SCLC) conduction in the presence of deep-level states.

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SCLC conduction phenomena encountered frequently in insulators [5], have been observed in wide bandgap semiconductors such as 6H a–SiC [6,7], b-SiC [8] and wurtzite AlN [9]. This current flow mechanism has also been reported in semi-insulating GaAs [10]. Up to now, to our knowledge, SCLC conduction in III-nitrides has been rarely reported in the literature [4,9,11]. Maruska and Stevenson [12] once considered SCLC as a possible conduction mechanism in their GaN light-emitting diodes, which was excluded finally. Nevertheless, Fedison et al. [11] found space-charge-limited currents in their Mg-doped GaN p+n junction diodes at large forward and reverse bias. As mentioned above, Hall et al. [4] attributed the abnormality of the I–V characteristics for their GaN rectifying contacts to SCLC conduction. Therefore, special caution should be paid while one analyzes the electronic properties of III-nitride materials and device structures. In the present work, we prepared GaN Schottky UV photodetectors on sapphire substrates by using MOCVD growth technique. Current–voltage (I–V) and DLTS measurements on these Schottky diodes have been carried out. Based on the results, a discussion regarding the possible current conduction mechanisms that dominate the I–V characteristics and the estimation of the trap densities in our materials will be presented.

2. Experimental The Schottky diodes studied in this work were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on basal plane sapphire substrates. Fig. 1 is the schematic diagram of the Schottky diodes. The structures consisted of 1.0 lm thick Si doped n-GaN films with a donor concentration close to 5 · 1018 cm
3, 0.6 lm thick unintentionally doped semiinsulating (SI) GaN layers with electron concentration about 1017 cm
3. Ti/Al/Ti/Au Ohmic and Ni/Au transparent Schottky contacts were sputter deposited to n-GaN and SI-GaN layers, respectively. The Schottky contacts were annealed in nitrogen at 500 C for 5 min, while no thermal treatment was made for the Ohmic contacts. Mesa structures were prepared by standard photolithography and dry etching. The area of the devices studied here was close to 10
3 cm2.

Fig. 1. Schematic diagram of the GaN Schottky diodes.

Dark current–voltage (I–V) characteristic measurements at room temperature (RT) were performed to electrically characterize the fabricated diodes. A computer controlled Keithley Model 6430 Sub-Femtoamp Remote Source Meter was used for these measurements. The DLTS measurements were performed with a fully automated set-up based on a C–V meter, pulse generator and a cryostat for changing the temperature in the range of 77–400 K.

3. Results and discussion Typical dark current–voltage linear plots of three GaN Schottky diodes fabricated from a GaN/sapphire wafer described above are shown in Fig. 2. Obviously, these diodes are rectifying with low leakage current in the range of 10–103 pA at
5 V. In the forward bias, Ôturn-onÕ voltages around 3 V were observed which were clearly large for Schottky diodes. In addition, kinks were observed in the forward bias I–V curves, as shown in the inset in Fig. 2. These kinks in the I–V curves reflected abrupt current changes, which will be discussed later. Fig. 3 illustrates the semi-logarithmic plots of the I–V characteristics in forward bias for the same Schottky diodes mentioned above. As shown in Fig. 3, there are three plateaus in the forward bias I–V curves indicating the deviations from the ideal Schottky diode. In particular, there is almost no well defined linear parts in the semi-logarithmic I–V characteristic for sample 3#. However, under moderate forward bias (1 V < voltage < 2 V), there exists a near linear region for 1# and 2# diodes. Assuming that this regime can be described by the thermionic emission theory, i.e., I / expðqV =nkT Þ

ð1Þ

where q is the electron charge, k is the Boltzmann constant, and T is the absolute temperature of the diode, then the diode ideality factor n obtained through linear fitting is >5. This value of n does not fall within the reasonable range of 1–2. Following this near linear regime in the semi-logarithmic plots abrupt change of current appeared at about 3 V, which is sample dependent. At higher voltages, the current seems to saturate. As thought conventionally, the series resistance effect appears to dominate the conduction process in this regime. However, the series resistances of the three diodes obtained through fitting are unreasonably large. In conclusion, the above features of the I–V characteristics suggest that other current transport mechanism is dominant, thus thermionic emission over the Schottky barrier is suppressed. To understand more clearly the electrical properties of these GaN Schottky diodes, their I–V characteristics were plotted on log–log scale. Fig. 4 illustrates the for-

X.M. Shen et al. / Solid-State Electronics 49 (2005) 847–852

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0.0015 -4

3.0x10

Dark current (A)

Dark current (A)

#

0.0010

1 # 2 # 3

-4

2.0x10

-4

1.0x10

0.0005

0.0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

Voltage (V) #

1 # 2 # 3

0.0000

-20

-15

-10

-5

0

5

10

Voltage (V) Fig. 2. Linear plots of dark current–voltage characteristics for GaN Schottky diodes.

0.01 1E-3 1E-4

Dark current (A)

1E-5 1E-6 1E-7 1E-8 1E-9 1E-10

#

1 # 2 3#

1E-11 1E-12 1E-13 1E-14 1E-15 0

2

4

6

Voltage (V) Fig. 3. Semilogarithmic plots of I–V characteristics for the same GaN Schottky diodes.

ward bias logarithmic I–V characteristics. At low-voltages (<1 V) an ohmic relationship, I / V, was observed for all the diodes investigated in this study. At V > 1 V the current rises quickly with a relationship of I / Vm (with m up to 22). Then, at about 3.0 V, the current rises nearly vertically which was followed by a square law dependence of current on voltage, I / V2. These phenomena are the very characteristics of spacecharge-limited current conduction in an insulator with

shallow and/or deep trapping centers and thermally generated carriers. As well known, the behavior of the injected carriers in a semiconductor through an ohmic contact is affected, to some extent, by the physical properties of the material in which the carriers are flowing. In the case of III-nitrides, the trapping centers created by impurities and defects will capture and thereby immobilize a fraction of the injected carriers, thus controlling the current trans-

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X.M. Shen et al. / Solid-State Electronics 49 (2005) 847–852 0.01 1E-3

TRAP-FREE SQUARE LAW

1E-4

Dark current (A)

1E-5 1E-6 1E-7 1E-8

V2

1E-9

V1

1E-10

1

1E-11 1E-12

OHM'S LAW

2 3

1E-13 1E-14 0.01

0.1

# # #

1

Voltage (V) Fig. 4. Log–log plots of I–V characteristics for the GaN Schottky diodes. The behavior under forward bias is suggestive of space-charge-limited current conduction.

port process. For this reason, the shape of the I–V characteristic is not only controlled by the contacts, but strongly dependent on the concentration and the energy distribution of trapping centers inside the specimen as well. As a bulk effect, SCLC conduction in solids (especially in insulators) has been reviewed in detail by Lampert and Mark [5]. They loosely defined materials with Eg 6 2 eV as semiconductors and those with Eg P 2 eV as insulators. That means, GaN (Eg = 3.4 eV) could be considered as an insulating material, depending on the concentration of doping. SCLC in solids is analogous to the process which governs the operation of a vacuum diode. The anode current in a vacuum diode is given by the ChildÕs Law, I / V3/2, while in solids the power law dependence of current on applied voltage would be somewhat different, I / Vm, where m varies with the injection level and is also related to the distribution of trapping centers. In most cases, three conduction regions in SCLC current–voltage characteristics will be observed [5]: (1) an ohmic region at relatively low-voltages where the current I is proportional to voltage V; (2) charge trapping region in which a square and/or a nearly vertical trap-filling-limited (TFL) sub-regions would follow the ohmic region; (3) finally the spacecharge-limited region where I proportional to V2. The above features of SCLC in solids are very typical according to LampertÕs simple theory in the case of one discrete trapping level. However, the assumption of single discrete energy for all the traps is an over simplification of the situation, since it is well known that the trapping sites would not be identical in nearest, next-

nearest environment and so on. In fact, the existence of two trapping levels distributed in energy around levels E1 and E2 (E1 < E2) simultaneously acting in the SCLC conduction mechanism has been observed in some semiconductor materials [13]. For the I–V characteristics presented here, they are considered as the case of two trapping levels and the thermal equilibrium Fermi level F0 lying between E1 and E2 [5,13]. In this case, the current follows OhmÕs law at low-voltages up to the first TFLvoltage, V1, which corresponds to the end of trap filling around E1, V1 can be expressed as the following [5] V 1 ’ qpt0;1 L2 =er e0

ð2Þ

where q is the electronic charge, er is the relative dielectric constant of the insulating material, e0 is the permittivity of vacuum, L is the thickness of the insulating material, and pt0,1 is the concentration of traps around E1 not occupied by electrons. At bias larger than V1, the current rises very quickly up to some current value at which the corresponding injection level is comparable to pt0,1. Thereafter the traps at E2 with a concentration of Nt2 dominate and the current changes slowly up to the second TFL-voltage, V2, which corresponds to the end of trap filling around E2, V2 can be expressed as the following [5] V 2 ’ qNt2 L2 =er e0

ð3Þ

where the quantities have the same meaning as in Eq. (2). At V2 the current rises nearly vertically to a value which merges gradually with the trap-free square law. As shown in Fig. 4, for the three diodes (and other diodes made from the same wafer, not shown here) V1

X.M. Shen et al. / Solid-State Electronics 49 (2005) 847–852

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30

DLTS Signal (a.u.)

25

Vb = -3V ET = 0.19eV

20

15

10

5 50

100

150

200

250

300

350

Temperature (K) Fig. 5. Representative DLTS spectrum for a GaN Schottky diode, measured at Vb =
3 V. The activation energy of the deep level is ET = 0.19 eV, and the concentration is about 4.65 · 1015 cm
3.

is almost the same, 0.9 V. Unlike V1, V2 is much more sample dependent and varies even for the same diode. For samples 1#, 2#, and 3#, V2 is 2.6, 3.0, and 3.3 V, respectively. The concentration of trapping centers can be estimated from Eqs. (2) and (3) and with following parameters: er ’ 10, L = 0.6 lm (supposing SCLC is mainly caused by the SI-GaN layer). For the three samples, the calculated concentration of unoccupied traps at E1, pt0,1, is about 1015 cm
3. The estimated values of Nt2 of samples 1#, 2#, and 3#, are 3.6 · 1015, 4.2 · 1015, and 4.6 · 1015 cm
3, respectively. Therefore, the lower limit of the concentration of the deep trapping centers in our samples is 1015 cm
3. The origin of these trapping centers are not clear at present and the determination of the positions of E1 and E2 needs further investigations such as I–V measurements at different temperatures (I–V–T). The activation energy ET of the deep levels for all the samples studied here is around 0.2 eV, which was reported in the literature [1]. A representative DLTS spectrum for a GaN Schottky diode is presented in Fig. 5. The density of the deep level with an activation energy value of ET =0.19 eV is about 4.65 · 1015 cm
3. Obviously, the concentration of the deep centers extracted from the DLTS data has the same order of magnitude as the SCLC estimation.

their I–V characteristics were non-ideal. The main features of the forward bias log–log I–V curves are indicative of space-charge-limited current (SCLC) conduction dominating the current transport in these GaN Schottky diodes. Thus, the expected thermionic emission process in these GaN Schottky diodes is suppressed due to SCLC. The lower limit of trapping center densities was estimated to be 1015 cm
3. The presence of SCLC may be related to the quality of Schottky contacts and the thickness of semi-insulating GaN layers adopted in the Schottky diodes. So, it can provide useful information for further optimization of the devices. On the other hand, SCLC measurements can be expected to probe the properties of deep levels in wide bandgap GaN–AlGaN compound semiconductors, as is the case with insulators in the presence of trapping centers.

Acknowledgements This work was supported partially by the Chinese National Natural Science Foundation (no. 69825107) and the China Postdoctoral Science Foundation (no. 2003034271).

References 4. Conclusions Unusual behavior of dark current–voltage (I–V) characteristics was observed in GaN Schottky diodes. Although these Schottky diodes were clearly rectifying,

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