Investigation and reduction of RF loss induced by Al diffusion at the AlN/Si(111) interface in GaN-based HEMT buffer stacks

Abstract

GaN-based high electron mobility transistors (HEMT) on Si (111) substrates have large potential for applications in the 5G telecommunication field. However, for this potential to be fully realized, all loss mechanisms need to be minimized. It is known that typical metal-organic chemical vapor deposition (MOCVD) processes used to grow the GaN epitaxial layers can cause considerable parasitic conductivity at the interface of the AlN nucleation layer to the high-resistivity Si substrate, leading to reduced gain and power added efficiency in amplifiers. Reducing this parasitic conductivity is hence of utmost importance to render GaN-on-Si a significant contributor to next-generation 5G power amplifier technology. In this work, we employ secondary ion mass spectroscopy, spreading resistance profiling and insertion loss measurements up to 28 GHz using coplanar waveguides fabricated on the epitaxial layer stacks to study the origin and characterize the parasitic conductivity. While a single heat-up process in an AIXTRON G5+ reactor chamber cleaned using Cl2 does not introduce any extra dopants in the Si substrate, the epitaxial growth of (Al,Ga)N-based HEMT buffer layer stacks leads to the diffusion of Al and, to a lower extent, Ga acceptors into the Si substrate. Optimization of the MOCVD process towards lower growth temperatures leads to a strong reduction of density of diffused acceptors. This reduction goes in line with a significant decrease of the insertion loss from 0.45 dB mm −1 to only 0.20 dB mm −1 at a frequency of 28 GHz.

Ultralow-Supersaturation Al Pretreatment toward Low Dislocation Density and Low Radio Frequency Loss GaN/AlN Epi-Stacks on High-Resistivity Si Substrates

“…25−28 In order to further confirm these results, transmission electron microscopy (TEM) experiments were performed. Panels (a and b) and (c and d) in Figure 1 show the bright-field TEM images under two-beam conditions with g = [0002] and g = [11][12][13][14][15][16][17][18][19][20] for sample A (with conventional pretreatment conditions) and sample E, respectively. It is again shown that both screw-and edge-type threading dislocations in sample E are significantly reduced compared to those in sample A.…”

“…The dislocation density, as well as the RF loss, is among the lowest level reported so far in the literature for GaN-on-Si, as shown in Figure 5d. [19][20][21]34,47 These results indicate the promising potential of the USAP technique and make it beneficial for GaN-on-Si RF applications.…”

“…22 Although these methods are effective in reducing the RF loss, they may degrade the crystal quality of the buffer stacks at the same time. [18][19][20]22,23 For instance, reducing the growth temperature is effective to suppress the diffusion and thus reduce the loss. However, at low growth temperatures, the AlN layer suffers from a high threading dislocation (TD) density and poor surface morphology due to the low migration ability of Al adatoms.…”

“…The latter is suggested to play a crucial role, − especially at high growth temperatures. In order to suppress the diffusion, various approaches have been attempted, such as reducing the growth temperature − and introducing a 3C-SiC layer or SiN x blocking layer . Although these methods are effective in reducing the RF loss, they may degrade the crystal quality of the buffer stacks at the same time. − ,, For instance, reducing the growth temperature is effective to suppress the diffusion and thus reduce the loss.…”

“…In order to suppress the diffusion, various approaches have been attempted, such as reducing the growth temperature − and introducing a 3C-SiC layer or SiN x blocking layer . Although these methods are effective in reducing the RF loss, they may degrade the crystal quality of the buffer stacks at the same time. − ,, For instance, reducing the growth temperature is effective to suppress the diffusion and thus reduce the loss. However, at low growth temperatures, the AlN layer suffers from a high threading dislocation (TD) density and poor surface morphology due to the low migration ability of Al adatoms. , The sequent stress management and the crystal quality of GaN would be degraded, leading to the degradation of the device performance. , Thus, an effective approach that can reduce the loss while maintaining the crystal quality of the epilayer at a low growth temperature is urgently demanded.…”