Resonant tunneling of charge carriers in InGaN/GaN superlattice

The result of studies of resonant tunnelling of charge carriers in InGaN/GaN unipolar structure is presented. Authors show that at temperatures below 150 K the multiple negative-differential resistance regions are observed on a reverse current-voltage characteristic which is typical for inhomogeneous distribution field for sample with 6 nm barrier thickness. At GaN barrier thickness 3 and 12 nm resonant tunneling were not observed.


Introduction
The InGaN/GaN superlattices (SLs) and multiple quantum wells (MQWs) are widely used as active regions or buffer layers in laser diodes, ligth-emitting diodes, receivers of visible and ultraviolet ranges, etc. As a rule, the electrical characteristics of these devices are investigated in details. However, the characteristics of InGaN/GaN SLs and MQWs are not studied intensively. The author's different works showed that low-temperature transport of electrons cannot be explained by conventional quasi-equilibrium drift-diffusion model [1].
Studies of current-voltage (I-V) characteristics of structures with InGaN/GaN multiple quantum wells used for analysis of transport mechanism of charge carriers. It is well known that resonant tunneling of charge carriers in superlattice can be detected by using I-V measurements. In this case the regions of negative-differential resistance (NDR) at I-V curves are observed in the experiment [2]. Previously, such NDR regions were measured for different material systems (AlGaAs/GaAs [3], AlGaN/GaN [4,5,6] etc.) This paper presents the results of experimental study of resonant tunneling in InGaN/GaN superlattice.

Experimental details
Experiments were carried out for unipolar structures grown by a MOCVD method. on the [0001] patterned sapphire substrate. As precursors of III-group elements the following metal organic compounds were used: trimethylindium In(CH 3 ) 3 , trimethylgallium Ga(CH 3 ) 3 , triethylgallium Ga(C 2 H 5 ) 3 . As nitrogen precursor an ammonia (NH 3 ) was used. For donor dopant (silicon) silicomethane was used. The structure consisted of a 4-µm-thick GaN nucleation layer, a 2-µm-thick Si-doped n-type GaN, doped n-type layer with In x Ga 1−x N/GaN MQWs, and 17-nm-thick Si-doped ntype GaN. The MQW layers comprised ten periods of an In 0.15 Ga 0.85 N well (2 nm) and a GaN barrier (6 nm). Additionally we investigated structures with GaN barrier thickness 3 and 12 nm. As a top contact we used Ni Schottky barrier. As an ohmic contact for buffer n-GaN layer we used fusing In contact of large area. Schematically investigated structure is shown in figure 1. In the experiment we measured the current-voltage characteristics in the temperature range T = 10 -300 K using a Janis cryostat and Keithley 2636A Source Meter. The measurements were carried out in a static and pulse mode to avoid overheating structures.

Experimental results
Initially we studied dependence of barrier height on temperature using n-GaN/Ni structure: where k -Boltzmann constant, T -temperature, A * -Richardson constant, j s -saturation current. In figure 2 the dependence of Schottky barrier height on temperature is shown. Drop height of the barrier is observed at temperatures below 200 K.   In the figures 5 and 6 experimental I-V characteristics for sample with 6 nm are shown. In the range from 0.5 to 0.7 V one or two regions of the NDR are observed on forward I-V characteristics which disappear when the temperature increases ( Figure 1). Such behavior of I-V characteristic is typical for the case of homogeneous electric field distribution in superlattice [2]. At temperatures below 150 K at above 0.7 V the multiple NDR regions are observed on a reverse I-V characteristic which is typical for inhomogeneous distribution field (Figure 2).
In both figures 5 and 6 it is shown that the numbers of NDR regions are decreased by growing up the temperature. However, at room temperature (T = 300 K) those NDR regions are not observed at all.

Conclusion
Thus, according to the results of this study, NDR regions were observed in the forward and reverse I-V curves for the InGaN/GaN SL with GaN barriers thickness of 3 nm. Found areas are observed at temperatures below 150-200 K. And at the same temperature reduction in the potential n-GaN/Ni barrier height occur.
Therefore it is explained the fact that NDR regions occur due to the resonant tunneling of charge carriers through the GaN barriers. By increasing the thickness of GaN barrier NDR regions do not occur. It can be associated with decreasing of tunneling probability of the charge carriers. By reducing the thickness of barriers a split of discrete levels in the miniband occurs. The detailed analysis of experimental results considers miniband vertical transport and resonant tunneling under condition of homogeneous and inhomogeneous electric field distribution in SLs [2] In this work we have shown the result which demands additional studies. Hopefully, in the next few years, we are going to have new data that will provide answers and guidance for our next steps in the investigation of transport change carries of InGaN/GaN SL.