Analyses of 2DEG characteristics in GaN HEMT with AlN/GaN superlattice as barrier layer grown by MOCVD
 Peiqiang Xu^{1},
 Yang Jiang^{1}Email author,
 Yao Chen^{1},
 Ziguang Ma^{1},
 Xiaoli Wang^{1},
 Zhen Deng^{1},
 Yan Li^{1},
 Haiqiang Jia^{1},
 Wenxin Wang^{1} and
 Hong Chen^{1}
DOI: 10.1186/1556276X7141
© Xu et al; licensee Springer. 2012
Received: 20 September 2011
Accepted: 20 February 2012
Published: 20 February 2012
Abstract
GaNbased highelectron mobility transistors (HEMTs) with AlN/GaN superlattices (SLs) (4 to 10 periods) as barriers were prepared on (0001) sapphire substrates. An innovative method of calculating the concentration of twodimensional electron gas (2DEG) was brought up when AlN/GaN SLs were used as barriers. With this method, the energy band structure of AlN/GaN SLs was analyzed, and it was found that the concentration of 2DEG is related to the thickness of AlN barrier and the thickness of the period; however, it is independent of the total thickness of the AlN/GaN SLs. In addition, we consider that the sheet carrier concentration in every SL period is equivalent and the 2DEG concentration measured by Hall effect is the average value in one SL period. The calculation result fitted well with the experimental data. So, we proposed that our method can be conveniently applied to calculate the 2DEG concentration of HEMT with the AlN/GaN SL barrier.
Introduction
The GaNbased highelectron mobility transistor (HEMT) is a promising research subject because of the expected advantages for the realization of electronic devices for highpower and hightemperature operations [1–5]. There are also applications as biosensors for detection of bacteria, DNA, and so on. The Al_{0.3}Ga_{0.7}N/AlN/GaN structures achieved a twodimensional electron gas (2DEG) mobility of 2,185 cm^{2}/V·s at room temperature with the carrier density of 1.1 × 10^{13} cm^{2} which was deposited by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate [6]. However, the relatively low carrier sheet density with AlGaN/GaN structures hinders the further development of device process. Although the sheet carrier density can be further improved by increasing the Al content in the ternary barrier layer, this increases the strength of polarization; on the other hand, increasing the Al content in the ternary layer may deteriorate the quality of AlGaN due to large lattice mismatch, and ternary alloy scattering in the heterostructure resulted in poor transport properties [7]. In contrast, the growth of AlN barrier layer on GaN channel enhances the transport properties. Introducing nanostructures into the HEMT epilayers can enhance the device performance. For example, recent reports have shown that by replacing the conventional AlGaN barrier layers with AlN/GaN superlattices composed of severalnanometerthick binary alloys, electric field induced by macroscopic polarization is much stronger, and the sheet carrier density is higher [8]. The AlN/GaN SLs have stronger polarization effect and larger conduction band offset than the heterostructure of AlGaN/GaN, and the SLs have deeper triangular quantum well.
The carriers in the heterointerface are confined twodimensionally and have stronger quantum effect than heterostructure of AlGaN/GaN as sheet carrier density, which makes more density of 2DEG and higher mobility in GaNbased HEMT device possible. Despite excellent transport properties, the described AlN/GaN structures have an obvious drawback: the 2DEG electron channel lies very close to the surface which makes it very sensitive to any process occurring at the surface of the sample. Compared with AlN as barrier, the 2DEG electron of HEMT with the AlN/GaN SL barrier has strong antijamming because it has several similar 2DEG electron channels which reduce any influence from surface by several times, and the sheet carrier density of HEMT with the SL barrier is much higher than AlGaN as barrier.
The purpose of this paper is to present detailed calculations about HEMT with the SLs as the barrier layers. There are many studies on growing the HEMT with SL barrier but there are few reports on calculations about the sheet carrier density of heterostructure with SLs as barrier. As the energy band of HEMT with SL barrier is analyzed, we assume that the 2DEG will be formed in every SL period if the thickness of AlN is beyond the critical thickness (d_{0}) which is the least limit thickness of AlN barrier to form the 2DEG. We also consider that the 2DEG density in each SL period is equal since the SLs have periodicity and tunnel impenetration. We deduce the equation which can be applied to calculate the 2DEG density in GaN/AlN/GaN heterostructures and calculated the 2DEG density with the data of our SLs, and at the same time, the calculated results were compared with experimental data.
Experimental details
Three groups of heterostructures were grown. One group consisted of different Al compositions, and the other two consisted of equivalent Al compositions. While the SLs in the second group had different period thicknesses, the last group had the same period (3.5 nm) but with different numbers of SLs (4 to 6 periods). Hall effect measurements were performed using the vander Pauw method on cleaved 6mm × 6mm squares using In/Ga alloy as ohmic contacts.
Results and discussion
where ε_{0} is vacuum permittivity, n_{s} is the sheet carrier density of HEMT, d_{0} is the critical thickness of AlN which can form 2DEG. If the thickness of AlN is smaller than d_{0}, it could not form the 2DEG. ε_{AlGaN} is the relative dielectric constant of AlGaN, d_{AlGaN} is the thickness of the barrier, eφ_{B} is the Schottky barriers of the gate contact on top of AlGaN, E_{F}(x) is the Fermi level with respect to the GaN conductionbandedge energy, and ΔE_{C} is the conduction band offset at the AlGaN/GaN interface where 2DEG forms.
The energy E_{D} of surface state is below the E_{F} if the thickness of AlN is less than d_{0}. With barrier thickness increasing, E_{F}E_{D} is decreasing. In addition, the energy E_{1}, the surface energy of conduction band, increases with increasing barrier thickness because there is a constant electric field in the AlN barrier due to the unscreened polarization field. At d_{0} (E_{F}E_{D} = 0), the donors at a free surface will begin to be ionized, and the PZ charge density at the AlN/GaN interface will start to be compensated by the 2DEG formed at the interface. The charge at the top of the AlN will be compensated by charged surface states for a free surface when the thickness of AlN barrier is beyond the critical thickness. At critical thickness (E_{F}E_{D} = 0), electrons are able to transfer from occupied surface state to empty conduction band states at the interface. The 2DEG density is increasing with increasing barrier thickness, while the E_{D} and E_{1} are constant and independent on the barrier thickness. The E_{1} is dependent on the material of the barrier layer.
where ${P}_{\mathsf{\text{SP}}}^{\mathsf{\text{AlN}}}$and ${P}_{\mathsf{\text{SP}}}^{\mathsf{\text{GaN}}}$are the spontaneous polarization of AlN and GaN, and ${P}_{\mathsf{\text{PZ}}}^{\mathsf{\text{AlN}}}$is the piezoelectric polarization of AlN buffer layer; $\frac{\sigma}{e}$ is the polarizationinduced charge density determined by the difference in the total spontaneous and piezoelectric polarization within AlN and GaN layers; n_{s} is the sheet carrier density; and d_{0} is the critical thickness of AlN which can form 2DEG.
where d_{1} and d_{2} are the thicknesses of AlN barrier and GaN layer, respectively; E_{F} is the Fermi level position with respect to the GaN conductionband edge; and ε_{1} and ε_{2} are the dielectric constants of AlN and GaN, respectively. According to Equation 6, the 2DEG density is dependent on both the Al mole fraction and the thickness of SL period but independent on the total thickness of SLs when the AlN/GaN SLs are used as the barrier layer.
In fact, the sheet carrier density measured using Hall coefficient is the average value of all SLs. It is difficult to calculate the 2DEG density in every SL period directly. The sandwich structure shown in Figure 4 (c) can be seen as one period in SL barrier. We consider that the 2DEG will be formed in the every SL period, and the sheet carrier density is equal in every SL period since the SLs have periodicity and tunnel impenetration. So, the n_{s} in each channel could be calculated using Equation 6. Therefore, we think that we can simply calculate 2DEG density of the outermost SL period simply to represent the 2DEG density in every SL.
As is shown in Figure 3, the red diamonds correspond to simulate value by means of Equation 6 where the surface barrier value e φ_{B} is fixed at 0.55 eV, which was obtained by fitting the first experimental data points in Figure 3a. Although there was no report about the eφ_{B} of In/Ga alloy on GaN as seen in Figure 3a, b, c, we can see that the simulate values are accordingly well with the experimental value; hence, we think that Equation 6 can be used to compute the 2DEG density.
However, as shown in Figure 3b, the simulate values highly contradict with the experimental values. If the thickness of SL is small, the energy band bending in each SL period should not be the same, and the sheet carrier density is not equal in every SL period. As the interface of heterostructure is about 5Å thick, the interfacial influence may become notable as the thickness of one SL period decreases which makes it more difficult to compute for the 2DEG density.
Conclusions
AlN/GaN SLs as the barrier of HEMT were grown on semiinsulated GaN, and the formation of 2DEG was researched particularly. The 2DEG concentration, characterized by Hall effect measurements, was found to be as a function of AlN thickness in SL period but independent on the total thickness of SLs. The calculation with our innovative method have proven these laws. During the process of calculation, we consider that the 2DEG would form in every SL period when the AlN barrier thickness in SL period was beyond the critical thickness, and concentration of the 2DEG was equivalent in each period. However, once the AlN barrier thickness in SL period is smaller than the critical thickness, the 2DEG would not be formed in any SL period according to the theory. Besides, the interfacial influence may become notable, which makes it much more difficult to calculate the density. The model we set has proved to be fitted well with the experimental result and can be used to calculate the 2DEG density in HEMTs with SL barriers.
Abbreviations
 2DEG:

twodimensional electron gas
 HEMT:

highelectron mobility transistor
 MOCVD:

metalorganic chemical vapor deposition
 PZ:

piezoelectric polarization
 SLs:

superlattices.
Declarations
Acknowledgements
Present research work was supported by the National High Technology Research and Development Program of China (grant nos. 2011AA03A112 and 2011AA03A106) and the National Nature Science Foundations (grant nos. 60890192, 50872146, and 60877006).
Authors’ Affiliations
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