Rare-earth Doped GaN – An Innovative Path Toward Areascalable Solid-state High Energy Lasers Without Thermal 

Distortion (2nd year) 

by Michael Wraback and Mark Dubinskiy 

ARL-TR-5213 June 2010 

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ARL-TR-5213 June 2010 

Rare-earth Doped GaN – An Innovative Path Toward Areascalable Solid-state High Energy Lasers Without Thermal 

Distortion (2nd year) 

Michael Wraback and Mark Dubinskiy 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Rare-earth Doped GaN – An Innovative Path Toward Area-scalable Solid-state 

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14. ABSTRACT 

In situ neodymium (Nd) doping of gallium nitride (GaN) and aluminum nitride (AlN) by plasma-assisted molecular beam 

epitaxy (PA-MBE) has been demonstrated for the first time. For GaN, Nd doping as high as ~8 at.% has been demonstrated, 

with no evidence of phase segregation identified by x-ray diffraction (XRD) for Nd up to ~1 at.%. The strongest roomtemperature luminescence was observed for a doping level between 0.1–1 at.%. The Stark energy levels of the three 

characteristic Nd emission multiplets were resolved by photoluminescence (PL) and photoluminescence excitation (PLE) 

spectroscopy. The enhanced substitutional doping at the gallium (Ga) site and low optical loss in waveguide structures 

suggests that GaN:Nd may have significant potential for use in simple, area-scalable, room-temperature, diode-pumped solidstate high energy lasers (HELs). Next-generation devices may be able to take advantage of the improved thermal conductivity 

of an AlN host. 

15. SUBJECT TERMS

GaN, AlN, Nd, high energy laser, thermal conductivity 

16. SECURITY CLASSIFICATION OF:

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19a. NAME OF RESPONSIBLE PERSON

Michael Wraback 

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iii 

Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Objective 1

2. Approach 1

3. Results 2

3.1 Growth of Nd Doped GaN by MBE ................................................................................2

3.2 Optical Studies of Nd Doped GaN ..................................................................................4

3.3 Optical Studies of Nd Doped AlN .................................................................................12

4. Conclusions 13

5. References 14

6. Transitions 16

6.1 Publications ...................................................................................................................16

6.2 Presentations ..................................................................................................................16

List of Symbols, Abbrviations, and Acronyms 18

Distribution List 19 
iv 

List of Figures 

Figure 1. (a) Nd atomic percent and PL intensity versus Nd cell temperature and (b) GaN 

transmission at 600 nm as a function of Nd concentration. The reduced sample 

transmission is due to the rough back side. ................................................................................3

Figure 2. X-ray data for GaN:Nd at 1 at% and 8 at% Nd in the GaN film. ....................................4

Figure 3. Nd at% obtained by SIMS as a function of Nd cell temperature for two Ga fluxes: 

Ga = 1.5×10–7 torr BEP (blue) and Ga= 3.5×10–7 torr BEP (red). ...........................................4

Figure 4. Room-temperature (300 K) and low-temperature (~30 K) PL spectra in logarithmic 

scale from GaN:Nd optically excited at 836 nm. Transitions from the lower and upper 

levels of the 4F3/2 excited state to the mth Stark level of the 4I9/2, 4I11/2, and 4I13/2 levels and 

RE-related (marked with an arrow) phonon sidebands are labeled. ..........................................6

Figure 5. Low-temperature (~30 K) PL spectra in logarithmic scale from GaN:Nd optically 

excited below and above the bandgap at 836 nm and 325 nm, respectively. Longitudinal 

optical (LO) and transverse optical (TO) phonon sidebands are labeled. Peaks marked 

with a downward arrow indicate plasma lines from the 325-nm laser. .....................................7

Figure 6. PL excitation spectra in logarithmic scale from GaN:Nd measured at the 1106 nm 

emission line at ~13 and ~150 K. Transitions from the lowest two levels of the 4I9/2

ground state to the mth Stark level of the 4F5/2, 2H9/2, 4F7/2, and 4S3/2 levels, as well as LO 

and TO phonon replicas are labeled. ..........................................................................................9

Figure 7. Optical loss at emission energy 1.12 eV (1106 nm) measured using the SES 

technique. The solid line is a fit of the experimental data (open circles) according to 

equation 1. ................................................................................................................................11

Figure 8. Output intensity versus excitation length measured at emission energy 1.12 eV 

(1106 nm). The solid line is a fit of the experimental data (open circles) according to 

equation 2. ................................................................................................................................12

Figure 9. PL from Nd doped AlN as a function of growth temperature and Al flux for 4F3/2

to 4

I11/2 transitions. ....................................................................................................................13

List of Tables 

Table 1. Energy of the crystal field-split levels of Nd3+ ions in GaN for the main 

incorporation site as determined by PL, PLE, and CEES (in parenthesis) measurements. .....10
v 

Acknowledgments 

We would like to thank Eric Readinger for growth and physical characterization of the 

neodymium (Nd) doped gallium nitride (GaN) and helpful discussions; Grace Metcalfe, Keith 

Graham, and Larry Merkle for optical characterization and helpful discussions; and John Gruber 

for theoretical modeling and helpful discussions. 
vi 

INTENTIONALLY LEFT BLANK. 
1 

1. Objective 

The objective of this effort was to develop a qualitatively new approach to highly scalable diodepumped solid-state lasers based on rare-earth (RE) neodymium (Nd3+) doping of gallium nitride 

(GaN), a high thermal conductivity material, with the goal of fully eliminating the bottleneck in 

the heat removal process associated with the low thermal conductivity of the gain medium 

compared to that of heat-sinking materials. 

2. Approach 

Laser-based directed energy weapons (DEWs) are important components for future Army 

missile defense systems. The diode-pumped, RE-doped, solid-state laser is a very promising path 

towards achieving a DEW-sufficient level of average power from a reasonably compact device, 

but the extreme pump power densities, combined with the inevitable non-radiative losses in the 

pump-lase process, introduce severe thermal loading in the gain medium. Regardless of the 

sophistication of heat removal technique and its efficiency, the gain medium itself is the 

bottleneck for non-distortive heat removal―simply due to low thermal conductivity of known 

gain media compared to that of heat-sinking material. The best known laser hosts, e.g., yttrium 

aluminum garnet (YAG), possess thermal conductivities (10–11 W/(m-K)) that are ~1.5 orders 

of magnitude lower than those of known heat-sinking materials. In order to eliminate this 

technical hurdle, an innovative gain medium―with thermal conductivity on the same order as 

copper (~390 W/(m-K))―has to be engineered. 

The approach pursued in this work is based on RE doping of GaN, in particular, Nd3+-doped 

GaN, with a room temperature (RT) thermal conductivity (131 to 200 W/(m-K)), which is 10 to 

15 times higher than that of Nd:YAG and 20 to 30 times higher than that of Nd: yttrium lithium 

fluoride (YLF). RE-doped GaN laser development, if successful, would be a breakthrough 

toward simple, area-scalable, room-temperature (as opposed to cryogenically cooled) solid-state 

high-energy lasers (HELs) with nearly zero thermal distortions, eliminating the need for 

sophisticated heat management. After Favennec et al. (1) demonstrated a reduction in thermal 

quenching in erbium (Er)-doped materials with increasing bandgap, interest in wide bandgap 

semiconductors as host materials expanded. In particular, wurtzite GaN has strong ionic bonds 

that can enhance the intra-4f n

 transition probability in the RE3+ ion with substitutional 

occupation of the Ga site. Light emission from GaN doped with europium (Eu), Er, 

praseodymium (Pr), thulium (Tm), ytterbium (Yb), Nd, and dysprosium (Dy) has been 

demonstrated by photoluminescence (PL), electroluminescence (EL), and/or 

cathodoluminescence (CL) (2, 3). As the RE dopant, Nd is an excellent candidate due to its 
2 

success in Nd-doped solid-state lasers, which have attained power levels higher than that from 

any other four-level material (4). Nd has been incorporated into GaN through ion-implantation 

(5) and reactive co-sputtering (6). Ion-implantation creates doping profiles and damage effects 

that often require a post implantation anneal due to the amorphization of the GaN lattice; 

however, annealing does not completely recover the damage. GaN ion implanted with Nd finds 

the majority of Nd3+ ions sitting on the Ga substitutional “site”; however, four other “sites” 

associated with defects are also identified (5). For reactive co-sputtering of GaN with RE atoms, 

a post-growth anneal (≥700 °C) in nitrogen is often necessary for PL emission (7–9). 

Although PL (5) and EL (7) from Nd-doped GaN have been observed, the Stark levels of the 4f

states have never been resolved, partially due to implantation-related damage of the host material 

and Nd3+ ions occupying multiple sites. In this work, in situ doping of GaN with Nd by plasmaassisted molecular beam epitaxy (PA-MBE) is demonstrated for the first time, with enhanced 

substantial doping at the Ga site (10). This growth technique enables the resolution of the Stark 

energy levels of the Nd3+ ion in GaN by PL and photoluminescence excitation (PLE) 

spectroscopy. The PLE spectroscopy also enables one to define an optimal pumping wavelength 

amenable to near-IR diode laser pumping that limits thermal losses. Since the GaN:Nd layers 

grown by MBE are thin (2–5 μm), a guided-wave architecture for edge-pumping the device 

(which provides the highest level of laser efficiency due to the pump and signal mode 

confinement within a crystalline-guided structure) has been designed. The successful 

implementation of this approach could lead to the development of the first diode pumped 

Nd3+:GaN laser. 

3. Results 

3.1 Growth of Nd Doped GaN by MBE 

GaN samples were grown on sapphire substrates with a fixed growth rate at two temperatures. 

The Nd cell temperature was varied from 850–1025 °C, and the Ga flux, measured as beam 

equivalent pressure (BEP), was varied from 9.8×10–6 to 5.6×10–7 torr. The secondary ion mass 

spectrometry (SIMS) profiling tracked the Ga and Nd signals and assumed the Ga signal was 

associated with a fully dense stoichiometric GaN film. The average Nd doping concentration, 

over the bulk of the film thickness, was converted to Nd atomic percent within the GaN matrix. 

A maximum Nd concentration of ~8 at.% was demonstrated (figure 1a). Initial work for the 

incorporation of Nd used nitrogen rich growth, for which the Nd and Ga had little competition 

for the same lattice site, as excess nitrogen was always present (10). These samples produced 

significant PL intensities of the Nd 4f transitions, but only showed limited PL output with low 

Nd atomic percentages and when Nd atomic percentages were greater than 1. The epilayers with 

high atomic percent Nd (>1) had extra peaks in the x-ray diffraction (XRD) data, and the 

transmission and reflectance (not shown) data showed a diminished intensity (figure 1b), 
3 

possibly due to Nd clustering or the formation of a second phase. Figure 2 shows typical ω-2θ xray scans about the (0002) GaN peak that include the (0006) sapphire peak. The high angle 

shoulder on the GaN peak (17.27°) is the aluminum nitride (AlN) buffer layer. The typical x-ray 

rocking curve (XRC) about the GaN (0002) reflection yields a full width at half maximum 

(FWHM) of ~1400 arcsec, which is consistent with the use of nitrogen rich growth conditions 

shown to be favorable for RE optical emission from the GaN matrix (11). The FWHM of the 

XRC data for these nitrogen rich films implies a high defect density not ideal for thermal 

properties (12). Subsequent growths were carried out under varying Ga flux conditions, with 

significant improvement in crystal quality observed under Ga rich conditions (x-ray data not 

shown). Figure 3 shows Nd concentration obtained from SIMS measurements as a function of 

Nd cell temperature for two different Ga fluxes during growth. At higher Ga flux, one needs to 

increase the Nd cell temperature, and therefore the Nd flux, to attain the same amount of Nd in 

the film. This result suggests that Ga rich growth potentially leads to less Nd incorporation 

within the matrix, possibly due to the increased competition for that lattice position. Since the 

maximum Nd flux is limited by the cell design, one possible solution may involve growing in a 

more nitrogen rich regime using low dislocation GaN substrates to maintain the crystal quality of 

the GaN:Nd epilayers. 

8E-4 8.4E-4 8.8E-4

10-2

10-1

100

101

1

10

100

1000 950 900 850

RBS

 SIMS

PL 915 nm

Neodymium Atomic Percent

1/Temperature (K-1)

 PL Intensity (a.u.)

 Nd Cell Temperature (°C) 1000 980 960 940 920 900 880 860 840

7.8E-4 8E-4 8.2E-4 8.4E-4 8.6E-4 8.8E-4 9E-4

1E-3

0.01

0.1

1

10

SIMS Data

Transmission Data

1/Nd Cell Temperature (K-1)

Neodymium Atomic Percent

0.02

0.04

0.06

0.08

0.10

Transmission

Nd Cell Temperature (°C)

 (a) (b)

Figure 1. (a) Nd atomic percent and PL intensity vs. Nd cell temperature and (b) GaN transmission at 600 nm as a 

function of Nd concentration. The reduced sample transmission is due to the rough back side. 
4 

14 16 18 20 22

101

102

103

104

105

106

Counts (a.u.)

Theta (degrees)

 GaN:Nd ~1 at%

 GaN:Nd ~8 at%

 XRC GaN:Nd ~1 at% sapphire

AlN

buffer

GaN

Unidentified

Phase

Figure 2. X-ray data for GaN:Nd at 1 and 8 at.% Nd in the GaN film. 

Figure 3. Nd at% obtained by SIMS as a function of Nd 

cell temperature for two Ga fluxes: Ga = 1.5×10–7 torr 

beam equivalent pressure (blue) and Ga= 3.5×10–7 torr 

BEP (red). 

3.2 Optical Studies of Nd Doped GaN 

The GaN:Nd samples were mounted in a closed-loop helium cryostat system for temperature 

dependent PL and PLE measurements. A Nd: yttrium orthovanadate (YVO4)-pumped continuous 

wave (CW) Ti:Sapphire laser tunable between 750 to 950 nm and with an excitation density of 

~20kW/cm2 and a CW helium-cadmium (HeCd) laser at 325 nm and with an excitation density 

of ~0.1kW/cm2

 were used for below and above bandgap excitation, respectively. The resulting 

luminescence was collected into a 1-m focal length spectrometer and focused onto a liquid 

nitrogen cooled germanium (Ge) detector. 
5 

PL spectra from GaN samples doped with various Nd concentrations show similar spectral 

features. Figure 4 shows a typical PL spectrum at low temperature (~30 K) from Nd-doped GaN. 

Strong emission due to transitions from the 4

F3/2 excited state to the 4I9/2, 4I11/2, and 4I13/2

manifolds in the energy (wavelength) range 1.30–1.36 eV (910–950 nm), 1.08–1.13 eV (1100–

1150 nm), 0.84–0.89 eV (1400–1480 nm), respectively, are observed. These Nd transitions are 

red shifted with respect to the same transitions in a Nd:YAG matrix due to the difference in the 

GaN crystal field. Also, because of the crystal field experienced by substitutional Nd-ions at Ga 

sites, there are a total of J+1/2 Stark sublevels in each manifold, where J is the total angular 

momentum. The PL peaks appear in pairs, which are labeled in figure 4 by the integer m that 

corresponds to the transition from the 4F3/2 doublet to the mth Stark component of the 4I9/2, 4I11/2, 

and 4I13/2 manifolds of the Nd3+ ion. The lower (higher) energy PL peak of each pair corresponds 

to transitions from the ground state (first excited state) Stark level of the 4F3/2 excited state. The 

peaks in each pair are separated by 4.1 meV, indicating the splitting energy of the 4F3/2 doublet. 

The small size of this splitting allows thermal activation of the upper sublevel making the 

corresponding emission transitions observable even at the lowest temperatures used in our 

studies. The most intense emission peaks are observed from transitions to the 4

I11/2 manifold, 

with the strongest emission line at 1.12 eV (1106 nm) attributed to the transition from the m = 0 

Stark level of the 4F3/2 state to the m = 0 Stark level of the 4I11/2 manifold. Compared to roomtemperature PL spectra, the low-temperature spectra show relatively small (<1 meV) or no shifts 

in peak energy position, consistent with the 4f shell being well shielded from the host material 

(13). The PL intensity of the 1106 nm peak for direct pumping of the 4F3/2→4I9/2 transition 

(915 nm) at room temperature increases when increasing Nd up to ~0.5 at.% (figure 1). At 

higher Nd concentrations, the decrease in PL intensity may be linked to the diminished 

transmittance and possibly to quenching associated with the closer proximity of Nd atoms (14), 

as suggested by the additional XRD peak in figure 2. This possibility of quenching is also 

consistent with the observation of non-exponential fluorescence decay following pulsed 

excitation. 
6 

Figure 4. Room-temperature (300 K) and low-temperature (~30 K) PL spectra 

in logarithmic scale from GaN:Nd optically excited at 836 nm. 

Transitions from the lower and upper levels of the 4

F3/2 excited state 

to the mth Stark level of the 4

I9/2, 4I11/2, and 4I13/2 levels and RErelated (marked with an arrow) phonon sidebands are labeled. 

Figure 5 shows the PL spectra in logarithmic scale from GaN:Nd optically excited below and 

above the bandgap at 836 and 325 nm, respectively. The above bandgap pump creates electrons 

in the GaN conduction band that relax primarily through non-radiative transitions, presumably 

through defects, to the upper Nd bands, eventually ending up in the 4F3/2 band. In many REdoped GaN materials, these defects aid in electron transfer to the RE atom, often leading to 

stronger PL emission from multiple “sites” as described by O’Donnell et al. (15). Moreover, 
7 

when Nd ions occupy multiple sites within the GaN matrix, the PL for above and below bandgap 

excitation can reveal markedly different spectra. The extra peaks marked by a downward arrow 

in the 325 nm spectrum in figure 5 indicate plasma lines from the 325 nm laser. Non-laserrelated peaks that only appear in the above bandgap pumping PL are shaded. Disregarding the 

plasma lines, the PL spectra from GaN:Nd excited at 836 and 325 nm are nearly identical. These 

results indicate that the majority of Nd atoms sit on the same “site” within the GaN matrix, with 

much less influence of defect assisted transitions involving multiple “sites.” 

Figure 5. Low-temperature (~30 K) PL spectra in logarithmic scale from 

GaN:Nd optically excited below and above the bandgap at 836 

and 325 nm, respectively. Longitudinal optical (LO) and transverse 

optical (TO) phonon sidebands are labeled. Peaks marked with a 

downward arrow indicate plasma lines from the 325-nm laser. 
8 

In addition to the 4

F3/2 4I9/2, 4I11/2, and 4I13/2 transitions, weak peaks shifted lower in energy 

from the main peaks by 90, 64, and 11 meV are observed. The 90- and 64-meV shifts (see 

figure 5) agree well with the GaN LO and TO phonon energies (16), respectively, and are 

attributed to electron-phonon coupling, as observed in other RE-doped GaN materials (17). The 

11-meV mode (see figure 4) is not observed in Raman measurements of the bulk crystal and is 

hence assigned to a localized mode that involves the Nd ion. A mode of similar energy (12 meV) 

has recently been found in the excitation-emission spectra of Eu-doped samples (18). From the 

intensity ratio of the phonon sideband peak to the relevant zero-phonon peak, we can 

estimate the electron-phonon coupling strength to be quite weak, having a Huang-Rhys (19) 

factor S of <0.1. 

Transitions from the 4

I9/2 ground state to the upper states 4F5/2, 2H9/2, 4F7/2, and 4S3/2, between 

1.46–1.54 eV (805–850 nm) and 1.58–1.62 eV (765–785 nm) are observed in PLE spectra shown 

in figure 6. After excitation into the upper state, the ions relax into the 4

F3/2 state, from which 

they can radiatively decay back into the ground state. Figure 6 displays the PLE spectra detected 

at the emission energy 1.12 eV (1106 nm) at ~13 and ~150 K. We compare the temperature 

dependent PLE spectra and assume a Boltzmann distribution of the electron population in the 

4

F3/2 manifold to help identify the excitation peaks. At low temperature, we attribute the 

excitation peaks to transitions from the ground state and the first thermally excited state of the 

4

I9/2 level to the mth Stark level of the 4F5/2, 2H9/2, 4F7/2, and 4S3/2 upper states. This assignment is 

consistent with the splitting found in the PL spectra between the m = 0 and the m = 1 Stark level 

of the 4I9/2 manifold. PLE spectra detected at different wavelengths show the same peaks with 

similar intensity ratios. The strongest emission occurs at an excitation energy of 1.48 eV 

(836 nm). Again, phonon sidebands can be observed, which are shifted higher in energy by 

amounts related to the LO and TO phonons of the host material and RE-related phonons, just as 

observed for the PL spectra and hence confirming our assignment. Table 1 summarizes the 

crystal field-split level energies of Nd3+ ions in GaN (20). Additional information about 

transitions from the 4

I9/2 ground state to the upper states 2G7/2 and 4G5/2 between 1.98–2.07 eV 

(600–630 nm) was obtained from combined excitation-emission spectroscopy (CEES). 
9 

Figure 6. PL excitation spectra in logarithmic scale from GaN:Nd 

measured at the 1106 nm emission line at ~13 and ~150 K. 

Transitions from the lowest two levels of the 4

I9/2 ground state 

to the mth Stark level of the 4

F5/2, 2H9/2, 4F7/2, and 4S3/2 levels, 

as well as LO and TO phonon replicas are labeled. 
10 

Table 1. Energy of the crystal field-split levels of Nd3+ ions in GaN for the main incorporation site as 

determined by PL, PLE, and CEES measurements. 

Manifold No. of Stark Energy 

(meV) 

Manifold No. of Stark Energy 

(meV) 

4

I9/2 5 0 

5.9 

26.1 

30.4 

39.5 

4

F5/2, 2H9/2 8 1470.8 

1476.6 

1479.8 

1491.3 

1498.0 

1510.0 

1511.8 

1525.1 

4

I11/2 6 232.8 

236.8 

241.6 

244.7 

257.9 

263.5 

4

F7/2, 4S3/2 6 1594.2 

1596.9 

1599.3 

1602.6 

1608.5 

1611.2 

4

I13/2 7 477.2 

479.2 

483.2 

486.2 

501.0 

504.2 

508.4 

2

G7/2, 4G5/2 7 1987.7 

1998.5 

2022.8 

2050.4 

2051.1 

2056.3 

2061.5

4

F3/2 2 1353.5 

1357.7 

 

For initial optical loss and gain measurements, a GaN:Nd sample with a Nd concentration of 

0.03 at.% was etched into 750 x 460 m structures using standard photolithography. The optical 

loss was measured using the shifting excitation spot (SES) technique (21). Optical gain was 

measured using the variable stripe length (VSL) method (22). Both loss and gain measurements 

were performed with above bandgap excitation at room temperature. The internal loss coefficient 

 was measured at the strongest emission energy 1.12 eV (1106 nm). The optical loss is related 

to the output intensity I by the following equation: 

d

o I I e  (1) 

where Io is the output intensity at the excitation spot and d is the distance between the facet and 

the excitation spot. A linear fit to ln(I/Io) as a function of d gives an optical loss of  = 6.5 

±5 cm–1 (figure 7). This loss is >3 times smaller than that reported for Eu-doped GaN (20 cm–1) 

at an emission wavelength of 620 nm (23). 
11 

Figure 7. Optical loss at emission energy 1.12 eV (1106 nm) 

measured using the SES technique. The solid line is 

a fit of the experimental data (open circles) according 

to equation 1. 

The relationship between output intensity I and net modal gain gmod using the VSL method is 

given by the expression 

  1 mod

mod

  o g le

g

J I (2) 

where Jo is the spontaneous emission intensity emitted per unit length, l is the length of the 

excitation beam, and gmod is the net modal gain defined by gmod = gm–. The variables  and gm

are the waveguide confinement factor and gain of the material, respectively. 

Figure 8 shows the output intensity as a function of excitation length. By fitting the experimental 

data with equation 2, we obtain gmod = –2 ±5 cm–1, indicating a small gain of the material gm. The 

small material gain may be due to the small Nd concentration in the waveguide structure. The 

discrepancy between the experimental data and the fit in figure 8 at short excitation lengths may 

be due to diffraction effects. More recently, 836-nm pumping experiments have been conducted 

on bars containing thin GaN:Nd layers of similar Nd at.%, but with aluminum gallium nitride 

(AlGaN) waveguide cladding layers. The same experiments have also been conducted on bars 

containing a SIMS stack of GaN:Nd layers with Nd at.% of 1.439, 0.057, and 0.202. The total 

thickness of this sample was similar to that of the sample previously investigated, and it also 

has no AlGaN waveguide cladding layers. No improvement in gain or loss has been achieved 

thus far. 
12 

Figure 8. Output intensity versus excitation length measured 

at emission energy 1.12 eV (1106 nm). The solid line 

is a fit of the experimental data (open circles) according 

to equation 2. 

3.3 Optical Studies of Nd Doped AlN 

While GaN may be a good choice as a host for RE ion laser media because of its high thermal 

conductivity, wider bandgap materials such as AlN could potentially be even better RE ion laser 

hosts because of their higher thermal conductivity than GaN (up to 340 W/(m-K) for AlN (24), 

or 1.5 to 3 times that of GaN). To this end, Nd doping and luminescence have been investigated 

for AlN. Figure 9 shows the first report of characteristic luminescence spectra for in situ doping 

of Nd in AlN for different growth temperatures and Al fluxes, focusing on transitions from the 

4

F3/2 excited state to the 4I11/2 manifold. For a constant Nd flux, luminescence intensity increases 

as the growth temperature is lowered. In addition, at a given growth temperature luminescence 

intensity increases with decreasing Al flux and constant Nd flux. This trend is seen most clearly 

in the 850 °C growth data (navy and magenta lines in the plot), for which using 1⁄2 the Al flux 

leads to a nearly three times increase in Nd PL intensity. This trend suggests that since Al binds 

with N more aggressively than Ga and Nd takes the Al or Ga site, a larger Al flux suppresses Nd 

incorporation more so than a comparable Ga flux. In addition, the PL peak positions are shifted 

slightly with respect to those for Nd in GaN. For example, the dominant PL peak in AlN:Nd is 

red shifted by ~1 nm relative to the 1107 nm peak in GaN:Nd. However, the next largest peak (at 

1111 nm in GaN:Nd) is red shifted by ~5 nm in the AlN:Nd spectra. These differences may be 

associated with the different crystal field experienced by the Nd in AlN, perhaps due to the larger 

strain associated with Nd taking the site of the smaller Al atom. 
13 

11000 11500

1

2

3

4

5

6

7

8

9

10

Photoluminescence (a.u.)

Wavelength (Angstrom)

 CAR 950C Al 2.7E-8

 CAR 950C Al 4.1E-8

 CAR 900C Al 2.8E-8

 CAR 850C Al 2.8E-8

 CAR 800C Al 2.8E-8

 CAR 850C Al 1.4E-8

Figure 9. PL from Nd doped AlN as a function of growth temperature 

and Al flux for 4

F3/2 to 4I11/2 transitions. 

4. Conclusions 

In conclusion, in situ Nd doping of GaN and AlN by PA-MBE has been demonstrated for the 

first time. For GaN, the Nd doping is controlled by the GaN growth conditions and the Nd 

effusion cell temperature. The Rutherford backscattering spectrometry (RBS) and SIMS data 

indicated Nd doping as high as ~8 at.% with no evidence of phase segregation identified by XRD 

for Nd up to ~1 at.%. Nd incorporation was found to reach a limit while maintaining crystal 

quality. Strong room-temperature luminescence corresponding to the three characteristic Nd 

emission multiplets was observed, with the Stark energy levels resolved by PL and PLE. 

Although the 4f electrons are well shielded from the host material, weak electron-phonon 

interactions are still observable. Spectral correlation of the three characteristic Nd emission 

multiplets for above (325 nm) and below (836 nm) GaN bandgap excitation imply enhanced 

substitutional doping at the Ga site. The highest room-temperature PL intensities corresponded 

to a doping level between 0.1–1 at.%. The enhanced substitutional doping at the Ga site and low 

optical loss in waveguide structures suggests that GaN:Nd with a high enough Nd concentration 

and number of Nd atoms, low enough structural defect density, and a high quality laser resonator 

with cleaved facets may have significant potential for use in simple, area-scalable, roomtemperature, diode-pumped solid-state HELs. Next-generation devices may be able to take 

advantage of the improved thermal conductivity of an AlN host. 
14 

5. References 

1. Favennec, P. N.; L’Haridon, H.; Salvi, M.; Moutonnet, D.; Leguillou, Y. Electronic Lett.

1989, 25, 718. 

2. Steckl, A. J.; Zavada, J. M. Mater. Res. Bull. 1999, 24, 16. 

3. Steckl, A. J.; Zavada, J. M. Mater. Res. Bull. 1999, 24, 33. 

4. Koechner, W. Solid-State Laser Engineering; (Springer, Berlin), 5th ed., pg. 37, 1999.

5. Kim, S.; Rhee, S. J.; Li, X.; Coleman, J. J.; Bishop, S. G. Phys. Rev. 1998, B57, 14588. 

6. Kim, J. H.; Davidson, M. R.; Holloway, P. H. Appl. Phys. Lett. 2003, 83, 4746.

7. Kim, J. H.; Holloway, P. H. Appl. Phys. Lett. 2004, 85, 1689. 

8. Kim, J. H.; Holloway, P. H. J. Appl. Phys. 2004, 95, 4787. 

9. Kim, J. H.; Shepherd, N.; Davidson, M.; Holloway, P. H. Appl. Phys. Lett. 2003, 83, 641.

10. Readinger, E. D.; Metcalfe, G. D.; Shen, H.; Wraback, M. Appl. Phys. Lett. 2008, 92, 

061108.

11. Steckl, A. J.; Heikenfeld, J. C.; Lee, D. S.; Garter, M. J.; Baker, C. C.; Wang, Y. Q.; Jones, 

R. IEEE J. Sel. Top. Quantum Electron. 2002, 8, 749.

12. Zou, J.; Kutchetkov, D.; Balandin, A. A.; Florescu, D. I.; Pollak, Fred H. J. Appl. Phys.

2002, 92, 2534.

13. Silkowski, E.; Yeo, Y. K.; Hengehold, R. L.; Goldenberg, B.; Pomrenke, G. S. Rare Earth 

Doped Semiconductors II Symposium 1996, 69.

14. Vanuiter, L. G.; Johnson, L. F. J. Chem. Phys. 1966, 44, 3514.

15. O’Donnell, K. P.; Hourahine, B. Eur. Phys. J.-Appl. Phys. 2006, 36, 91.

16. Siegle, H.; Kaczmarczyk, G.; Filippidis, L.; Litvinchuk, A. P.; Hoffmann, A.; Thomsen, C. 

Phys. Rev. 1997, B 55, 7000. 

17. Dierolf, V.; Sandmann, C.; Zavada, J.; Chow, P.; Hertog, B. J. Appl. Phys. 2004, 95, 5464. 

18. Dierolf, V.; Fleischman, Z.; Sandmann, C.; Wakahara, A.; Fujiwara, T.; Munasinghe, C.; 

Steckl, A. Mater. Res. Soc. Symp. Proc. 2005, 3.6.1, 866. 

19. Vanuiter, L. G.; Johnson, L. F. J. Chem. Phys. 1966, 44, 3514. 
15 

20. Metcalfe, G. D.; Readinger, E. D.; Shen, H.; Woodward, N. T.; Dierolf, V.; Wraback, M. J. 

Appl. Phys. 2009, 105, 053101. 

21. Dal Negro, L.; Bettotti, P.; Cazzanelli, M.; Pacifici, D.; Pavesi, L. Opts. Commun. 2004, 

229, 337.

22. Shaklee, K. L.; Nahory, R. E.; Leheny, R. F. J. Lumin. 1973, 7, 284.

23. Park, J. H.; Steckl, A. J. Appl. Phys. Lett. 2004, 85, 4588.

24. Perry, P. B.; Rutz, R.F. Appl. Phys. Lett. 1978, 33 319. 
16 

6. Transitions 

6.1 Publications 

1. Readinger, E. D.; Metcalfe, G. D.; Shen, H.; Wraback, M. GaN Doped with Neodymium 

by Plasma-assisted Molecular Beam Epitaxy. Appl. Phys. Lett. 2008, 92, 061108. 

2. Metcalfe, Grace D.; Readinger, Eric D.; Shen, Hongen; Woodward, Nathaniel T.; Dierolf, 

Volkmar; Wraback, Michael. Crystal-field Split Levels of Nd3+ Ions in GaN Measured by 

Luminescence Spectroscopy. J. of Applied Physics 2009, 105, 053101. 

3. Metcalfe, Grace D.; Readinger, Eric D.; Shen, Hongen; Woodward, Nathaniel T.; Dierolf, 

Volkmar; Wraback, Michael. Energy Levels of Nd3+ Ions in GaN. Physica Status Solidi 

(c) 2009, 6, S671. 

4. Readinger, Eric; Metcalfe, Grace; Shen, Hongen; Woodward, Nate; Jha, Naveen; Dierolf, 

Volkmar; Wraback, Michael. GaN Doped with Neodymium by Plasma-Assisted 

Molecular Beam Epitaxy for Potential Lasing Applications. In Rare-Earth Doping of 

Advanced Materials for Photonic Applications, Dierolf, V.; Fujiwara, Y.; Hommerich, U.; 

Ruterana, P.; Zavada, J., eds., Mater. Res. Soc. Symp. Proc. Vol. 1111, Warrendale, PA, 

2009, 1111-D-02-04 (invited paper). 

5. Dierolf, Volkmar; Woodward, Nate; Jha, Naveen; Metcalfe, Grace; Readinger, Eric; 

Wraback, Michael. Spatially Resolved Site Selective Optical Spectroscopy on Nd Doped 

GaN Epitaxial Layers. In Rare-Earth Doping of Advanced Materials for Photonic 

Applications, edited by Dierolf, V.; Fujiwara, Y.; Hommerich, U.; Ruterana, P.; Zavada, J., 

eds., Mater. Res. Soc. Symp. Proc, Vol. 1111, Warrendale, PA, 2009, 1111-D-02-06. 

6.2 Presentations 

1. Readinger, Eric D.; Metcalfe, Grace D.; Shen, Paul; Wraback, Michael; Jha, Naveen; 

Woodward, Nathaniel; Dierolf, Volkmar. Plasma-assisted Molecular Beam Epitaxial 

Growth of (Al)GaN Doped with Neodymium for Potential Lasing Applications. Impurity 

Based Electroluminescent Devices and Materials, Tossa De Mar, Spain, October 2009.

2. Metcalfe, G. D.; Readinger, E. D.; Shen, H.; Woodward, N. T.; Dierolf, V.; Wraback, M. 

Energy levels of Nd3+ ions in GaN. International Workshop on Nitride Semiconductors, 

Montreaux, Switzerland, October 2008.
17 

3. Readinger, Eric; Metcalfe, Grace; Shen, Hongen; Woodward, Nate; Jha, Naveen; Dierolf, 

Volkmar; Wraback, Michael. GaN Doped with Neodymium by Plasma-Assisted 

Molecular Beam Epitaxy for Potential Lasing Applications. MRS Fall Meeting, Boston, 

MA, December 2008 (invited).

4. Dierolf, Volkmar; Woodward, Nate; Jha, Naveen; Metcalfe, Grace; Readinger, Eric; 

Wraback, Michael. Spatially Resolved Site Selective Optical Spectroscopy on Nd Doped 

GaN Epitaxial Layers. MRS Fall Meeting, Boston, MA, December 2008. 
18 

List of Symbols, Abbreviations, and Acronyms 

AlGaN aluminum gallium nitride 

AlN aluminum nitride 

BEP beam equivalent pressure 

CEES combined excitation-emission spectroscopy 

CL cathodoluminescence 

CW continuous wave 

DEW directed energy weapon 

Dy dysprosium 

EL electroluminescence 

Er erbium 

Eu europium 

FWHM full width at half maximum 

GaN gallium nitride 

Ge germanium 

HeCd helium-cadmium 

HEL high-energy laser 

LO longitudinal optical 

Nd neodymium 

PA-MBE plasma-assisted molecular beam epitaxy 

PL photoluminescence 

PLE photoluminescence excitation 

Pr praseodymium 

RBS Rutherford backscattering spectrometry 

RE rare earth 
19 

RT room temperature 

SES shifting excitation spot 

SIMS secondary ion mass spectrometry 

Tm thulium 

TO transverse optical 

VSL variable stripe length 

XRC x-ray rocking curve 

XRD x-ray diffraction 

YAG yttrium aluminum garnet 

Yb ytterbium 

YLF yttrium lithium fluoride 

YVO4 yttrium orthovanadate 
20 

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