# Carrier density analysis in stressed n-doped GaN on sapphire layers

Ndembi Ignoumba-Ignoumba (Ph.D. student), Camille Sonneville, Eric Frayssinet, Adrien Bidaud, Florian Bartoli, Yvon Cordier, Farid Medjdoub, Dominique Planson, Cyril Buttay

<figure><img src="image_1.png" /></figure>

Contact : ndembi.ignoumba-ignoumba@insa-lyon.fr

<figure><img src="image_2.png" /></figure>

## CONTEXT

* This work is part of VERTIGO project, which is one of the PEPR Electronic projects launched under the France 2030 Program.

* The aim of VERTIGO is to produce high-power vertical GaN-based transistors on silicon substrates demonstrating BV > 1.2 kV and a current of 50 A.

## GOALS OF THIS WORK

To synthetize and characterize thick GaN epitaxial layers (> 5 µm) on sapphire and silicon substrates in order to optimize their quality in terms of :

I. Homogeneous n-carrier density [10¹⁵ - 10¹⁶] cm⁻³

II. Homogeneous low biaxial stress

III. Homogeneous dislocations density [10⁸ - 10⁹] cm⁻²

## EXPERIMENTS

### GaN/sapphire epitaxial layers

Schottky diode structure

* Thick GaN epilayers are successfully grown by MOCVD on 2-inch sapphire substrates.

* n-carrier densities are ranging [1 - 6.5] · 10¹⁵ cm⁻³ as measured by mercury probe C-V technique with silicon as a dopant.

<figure><img src="image_3.png" /></figure>

iemn
Institut d'Electronique, de Microélectronique et de Nanotechnologie
UMR CNRS 8520

* Semi-vertical Schottky and p-n diodes structures are obtained by dry etching.

<figure><img src="image_4.png" /></figure>

p-n diode structure

<figure><img src="image_5.png" /></figure>

### Surface characterization

* Overall, smooth surfaces were synthetized.

* Mixed and edges dislocations were identified.

<figure><img src="image_6.png" /></figure>

Major surface defects were identified such as particles, holes, micro steps and edge discontinuity.

<figure><img src="image_7.png" /></figure>

## Stress and n-carrier density analysis by Raman spectroscopy

### Raman spectroscopy principle

<figure><img src="image_8.png" /></figure>

$E_2^H$ and $A_1(LO)$ pic positions (respectively $\omega_{E_2^H}$ and $\omega_{A_1(LO)}$) increase linearly with biaxial stress and $\omega_{A_1(LO)}$ also increases with n-carrier density ($D_n$)¹,².

$$ \omega_{E_2^H} - \omega_{E_2^H}^0 = K_{E_2}^B \Delta\sigma_B \quad (1) $$

$$ \omega_{A_1(LO)} - \omega_{A_1(LO)}^0 = K_{A_1}^B \Delta\sigma_B + f(D_n) \quad (2) $$

### Characterization on entire wafers before etching

Compressive stress

<figure><img src="image_9.png" /></figure>

* $\omega_{E_2^H}$ is stable across the entire wafers except at the edges, indicating homogeneous biaxial stress and free-edge relaxation effect.

* $\omega_{A_1(LO)}$ locally decreases and increases, indicating inhomogeneous n-carrier density, most likely due to carbon and oxygen impurities.

### Characterization on diodes structures after etching

<figure><img src="image_10.png" /></figure>

* Free-edge relaxation effects are more present after etching.

* $\omega_{A_1(LO)}$ and $\omega_{E_2^H}$ are found to be strongly correlated.

<figure><img src="image_11.png" /></figure>

* $\omega_{A_1(LO)}$ can be linearly correlated to $\omega_{E_2^H}$ through biaxial stress while the intercept depends on $D_n$ (Eqs (1) & (2)) :

$$ \omega_{A_1(LO)} = \frac{K_{A_1}^B}{K_{E_2}^B} \omega_{E_2^H} + f'(D_n) \quad (3) $$

Experimentally : $\frac{K_{A_1}^B}{K_{E_2}^B} \sim 0.78$.

Literature : [0.28 - 0.80]³

* Mean compressive stress increases with the diameter of the diode.

## FUTURE WORK

* Evaluate the impact of n-carrier density inhomogeneities, major surface defects, biaxial stress and dislocations density on I-V performances of GaN/sapphire Schottky and p-n diodes

* Conduct C-V, DLTS, SIMS on Schottky and p-n diodes in order to evaluate n-carrier density and to precisely identify the nature and the density of impurities

* Optimize GaN/sapphire growth parameters and device fabrication processes accordingly and adapt the methodology to GaN/Si Schottky and p-n diodes

## BIBLIOGRAPHY

1. F. Demangeot and J. Frandon, *Solid State Communications*, vol. 100, no. 4, 1996.

2. A. J. Eric N'Dohi et al., *AIP Advances*, vol. 12, no. 2, 2022, doi: 10.1063/5.0082860.

3. J.-M. Wagner and F. Bechstedt, *Phys. Rev. B*, vol. 66, n° 11, 2002, doi: 10.1103/PhysRevB.66.115202.

<figure><img src="image_12.png" /></figure>

PROGRAMME
DE RECHERCHE
ÉLECTRONIQUE

anr®
ANR-22-PEEL-0004

<figure><img src="image_13.png" /></figure>

INSA

INSTITUT NATIONAL
DES SCIENCES
APPLIQUÉES
LYON

<figure><img src="image_14.png" /></figure>

ÉCOLE
CENTRALE LYON

<figure><img src="image_15.png" /></figure>

Lyon 1