2-1 Introduction
Because many applications in sub-THz or THz band need compact, convenient, broadband THz sources with high output power. Some proposed applications include sub-THz sources, Gunn diodes, resonant tunneling diodes [1,2], and quantum cascade THz lasers [3-5]. Currently, several related products like THz image systems, THz spectroscopy kits, etc are now commercially available. These systems usually constructed with Ti:sapphire mode-locked laser (λ=800nm) and compact THz emitter modules. It’s attractive to replace these modules by compact photonic-transmitters (PTs), which are composed of an antenna and a high speed and power photodiode (PD) [6-9]. The PTs have the advantages of simplicity, room-temperature operation, tunable THz wavelength, and easy integrability with other semiconductor devices.
In order to further improve the maximum transmission distance in a sub-terahertz (sub-THz) IR communication links [10-12], it is necessary to have a PD which can sustain its high-speed performance and deliver high output power under intense optical pulse excitation. There are two major strategies to meet this challenge [13].
One is to distribute and make uniform the photocurrents along edge-coupled PDs by improving the structure of optical and electrical waveguides, for example, the velocity matched distributed photodetector (VMDP) [14] and evanescent coupled photodiode (ECPD) [15,16]; the other strategy involves minimizing the space-charge screening (SCS) effect [13, 17] in the photo-absorption volume. Under intense optical power illumination, the photo-generated carriers will induce a strong space-charge field,
25
screen out the external applied electrical field, and seriously limit the output saturation power of the PD. By increasing the drift-velocity of photo-generated carriers, such as with the structure of a uni-traveling carrier PD, excellent high-speed and high-power performance has been demonstrated [18].
Under 1.55 m long wavelength excitation, an InP/InGaAs based UTC -PD based photonic-transmitter has been shown to generate a continuous wave (CW) output power at 1.04THz of as high as 2.3 W [ 18]. In the case of an InP based UTC-PD operated under 0.8 m wavelength excitation, the incident photon will have enough photon energy to induce the absorption process in the InP based collector layer. The presence of photo-generated holes in the collector layer will degrade the high-power performance of the UTC-PD. We overcome this problem with our demonstrated high-speed GaAs/AlGaAs based UTC-PD [19], which is composed of a GaAs based p-type photo-absorption layer and an Al0.15Ga0.85As based collector layer.
Undesired photo-absorption under 800nm wavelength excitation is avoided with this device. We have also demonstrated a high-power photonic-transmitter that operates under 800nm optical short-pulse excitation [20].
It is also possible to minimize the SCS effect by decreasing the thickness of the depletion layer which has a direct effect on the carrier transit time, such as the structure of a partially depleted absorber photodiode [21]. However, PDs with thin depletion layers usually suffer from problems of low quantum efficiency and very limited RC bandwidth. In order to overcome the above-mentioned problems, we have demonstrated a p-i-n photodiode structure: the Separated-Transport-Recombination photodiode (STR-PD), which can greatly relieve the trade-off between output saturation power, quantum efficiency, and electrical bandwidth performance [22]. In the demonstrated GaAs based STR-PD, a
26
LTG-GaAs layer is adopted, which has an extremely short carrier lifetime (less than 1ps) [23], to serve as the recombination center in the active photo-absorption region.
The STR-PD exhibits superior electrical bandwidth performance under a higher output current regime without seriously sacrificing responsivity compared to that of the control p-i-n PD (with a pure intrinsic GaAs based photo-absorption layer) [25].
In this study, the integration of the STR-PD with a narrow-band slot-antenna to produce an STR-PD based photonic transmitter will be described previously in section 2-5. After that, we integrate two different kinds of high-power photodiodes (UTC-PD and STR-PD) with the same type of planar antenna (circular disk monopole antenna) to serve as photonic transmitters. We also compare their dynamic performance using the same THz time-domain spectroscopic system. These two different devices exhibit distinct dynamic behaviors and very different mechanisms of saturation.
Under high optical pulse energy excitation (~480pJ/pulse), the STR-PD based transmitter exhibits much a lower maximum average output photocurrent (1.2mA vs.
0.3mA) than that of the UTC-PD transmitter. The radiated electrical envelop-width (~50ps) and maximum peak-power (~9mW) of both devices are comparable. This indicates that although the DC responsivity of the recombination center in the STR-PD is degraded, the high-speed and output power performance of the device have been effectively improved and the DC component of the photocurrent eliminated. The smaller DC photocurrent implies that device-heating problems of STR-PD based transmitters during high-power operation will be decreased. The dynamic measurement results reveal that although the working principles for the high-power performance of the STR-PD and UTC-PD are totally different, both devices exhibit comparable and promising high-speed and high-power performance for applications in THz photonic transmitters.
27
2-2 Basic Theory
As implied in the name, photodiode is a device that transform optical signal to electrical signal. Thus, properties of bandwidth, transform efficiency and output saturated current are all aims of high speed and high efficiency PDs. [24] In order to achieve the above goals, not only geometrical structure but epi-layer structure is need to be designed well.
According to their geometry, structures of PDs could be classified into two types of vertically incidence (VPD) and edge coupled incidence (WGPD). [24] As for vertical incidence PDs, incident light can’t be absorbed completely after passing through absorption layer once due to the fixed thickness, thus quantum efficiency decrease seriously. This phenomenon could be improved by adding a reflector at the bottom of devices to help the number of times of light reflection. However, it could limit RC bandwidth of devices seriously. In the other side, edged incident photodiodes are provided with higher quantum efficiency because incident light could be absorbed while propagate in the waveguide at the same time., which structure is adopted in our designed components.
The design of travelling wave (TWPD).[25][26] are adopted to solve the problems mentioned above. Such design could not only reduce the influence of mismatch between optical velocity and electrical velocity but achieve best bandwidth through design of co-plane electrode shape which match with external resistance. In addition to geometrical structure, material of epi-layer structure also limit frequency response, quantum efficiency and saturation current of devices.
28
(a)
(b)
Figure 2- 1 (a)Vertically incident PD and (b) edge coupled incident PD
After carrier generation from absorption area, the distribution and transmission of carriers influence frequency response. In some designed epi-layer structure, carrier life time influence frequency response seriously.
Principal of traveling wave photodiode and frequency limitation
Traveling wave photodiode is a waveguide photodiode, which incident light inject into devices from edge side and the direction of light propagation and photo current is the same. The main limitation of bandwidth is caused by mismatch of optical and electrical velocity. [26] The best design is to match optical and electrical velocity of devices and to match external resistance with its electrode, which could
N
29
minimum reflection.
In the PIN ridge waveguide structure, the higher refraction index in the intrinsic layer could serve as transmission layer to guide light. It could be an absorption layer due to its lower band gap. Such kind of structure is like a microwave transmission line which light is absorbed while propagating in the waveguide As a result, an effective circuit of transmission line is used to design our structure which combing stripe transmission line and planar electrode than increase operation speed of devices.
Travelling Wave Photodiode effective circuit
In order to improve speed of devices, we have to design our devices by simplify devices structure as an effective circuit and then analysis them. A model with distributed current is used to explain speed limitation of devices. When light incident into our devices, carriers are generated in the inner absorption layer and accelerated by induced electrical field, which cause distributed photocurrent as shown in figure 2-2. According to theorem of transmission line, it will cause microwave signal parallel with light and collected in the output port.
As shown in figure 2-2, photo current is generated in the intrinsic layer while light is absorbed, which cause main capacitance of devices. Thus, value of capacitance is decided by thickness of waveguide and absorption layer directly. The equivalent circuit of active area of devices is shown in figure 2-3. In the theory of transmission line, intrinsic capacitance Ci is defined directly by the thickness of di and width w, which is written as
i
i d
C =εw
. Cto, Ct, Cb, Cbc is capacitance between metal and capacitance of semi-conductor, which has lower influence and shown in figure 2-3. On the other side, under reverse bias, photo current is generated while electrical field crossed in absorption area, which also induced the inductance of magnetic field Lm.
30
Figure 2- 2 Scheme of travelling wave photodiode
Induced photocurrent generated by carriers in absorption area would pass through conductor layer which is the both side of intrinsic layer. As a result, both two conductor layer are equivalent with resistance Zt and Zb. Photocurrent is generated from intrinsic layer and then transmission with connected gold wire finally. Because current distribution, it is generated not only the resistance between metal and semiconductor Zct, Zcb and metal inductance Lm, but also skin effect on metal
I
Figure 2- 3 An effective diagram of the device
31
surface. Such effect could be seen as parallel connection of resistance Zmt and conductance Gm as shown in figure 2-4.
Figure 2- 4(a)Equivalent circuit of photo diode and (b) simplified equivalent circuit of photo diode.
Therefore we may know by the transmission line definition that the part the
characteristic impedance is
Y Z0 = Z
,
The microwave propagation constant is
m m
m ZY
α
jβ
γ
= = + ⋅. Therefore after we obtain the part structure transmission electric properties, then simulate the frequency response of devices and to study its bandwidth performance.
2-3 Separated-Transport-Recombination Photodiode (STR-PD)
Principal and problem of epi-layer structure for traditional
photodiode
In the traditional photodiode structure, we proposed two traditional photodiodes, make the principle discussion and analyze the epi-layer structure separately, which may affect bandwidth limitation and output power seriously. The encountered
32
problem and induced bottleneck is introduced as follows:
Traditional PIN GaAs based Photodiode
The traditional PIN GaAs PD is refers to that its material of absorption layer is composed by using only GaAs, which may reduce RC bandwidth limitation by increasing thickness of absorption layer. However, such reconstruct increase carrier drift time and lower devices speed and saturation current. In the other side, if we decrease absorption layer to achieve higher saturation current, problems caused such as serious bandwidth limitation and lower quantum efficiency. Moreover, under high input power excitation, too much photo carrier generated may cause the speed slow electricity hole accumulates in inner absorption layer, which will form a built-in field.
[25-26] Such built-in field would screen external field and reduce devices speed and P-AlGaAs
N-AlGaAs
G a As
Figure 2- 5A schematic drawing of energy band and electric field distributed for traditional GaAs PD under high power excitation
33
output saturation current, which is so-called space charge screen effect. The trade-off problem between RC bandwidth limitation, carrier drift time and saturation current become a complex and hard test while design our devices.
Traditional PIN LT-GaAs based PD
Traditional PIN LTG-GaAs PD stands for its absorption layer are composed of by only low temperature grown GaAs. [24] Because there are lots of defects in LTG-GaAs, drift carrier generated will be captured and recombined, which means short carrier life time as shown in figure 2-8.[27] In other words, speed limitation of devices would transform from carrier drift time to carrier life time, this may improve devices speed a lot. By this method, trade off problem between capacitance bandwidth limitation and carrier drift time will be solved. It also improves space charge effect in this structure under higher input power excitation.
However, because LTG-GaAs in absorption layer is material with short carrier life time, most generated photo-carrier will be recombined and reduce quantum efficiency seriously. In the other side, there are much defects exists in absorption layer which may induce effect like heavy doping in absorption layer while applied external electrical field on the devices Thus depletion layer will exist in the junction layer between absorption layer and conduction layer. As a result, electrical field would be applied in conduction layer and lower internal electrical field in the absorption layer, which may limit carrier speed and limit speed and efficiency of devices.
34
In order to apply electrical field in the absorption area, higher applied voltage on the devices is needed. However, when we apply higher voltage on the absorption area, speed of devices is not e increased as excepted. This is because carrier life time
Normal Enhanced Electric field
Carrier will be trapped Out of shape
Carrier can tunnel off
LTG-GaAs
defects P-AlGaAs
N-AlGaAs
Figure 2- 6Traditional LTG-GaAs PD energy band diagram and electrical field distribution.
Figure 2- 7A scheme of change for trap energy band
35
increasing effect for LTG-GaAs, which means trap energy will change shape due to column effect under high applied voltage. [29-30] such effect may cause trapped carriers escaped from trap state in tuning mechanism, which means poor recombination effect and poor operation speed.
Separated transmission recombined travelling wave PD (STR-PD)
According to above two type traditional photodiode, we can learn trade off problems of speed, efficiency and saturation power will cause problem of design devices. Thus we propose a new type photo diode to solve the problem-- Separated transmission recombined travelling wave photo diode (STR-PD).
P-AlGaAs
N-AlGaAs LTG-GaAs
GaAs GaAs
Transport Layer
The position of absorption region with high electric field
Recombination center
The position of absorption region with weak electric field
Figure 2- 8Schematic diagram of energy band and electrical field for STR-PD.
36
We adopt travelling wave photo-diode type in PD’s structure and recombination center (LTG-GaAs) in absorption layer. The STR-PD is composed of inserted LTG-GaAs in intrinsic layer GaAs and shown in figure 2-10.
Principal of STR-PD
Because we insert recombination center (LTG-GaAs) in absorption area, total absorption layer thickness become thinker and thus capacitance smaller. Besides, due to carriers in recombination center with slower speed trapped by defects, we could replace carrier drift time limitation by carrier life time. The higher speed performance is expected for carrier trapped time << carrier life time. The key point is we solved trade off problem between RC value and carrier drift time by inserted LTG-GaAs in traditional PIN PD, which we can improve devices performance a lot.
P-AlGaAs
N-AlGaAs LTG-GaAs
GaAs GaAs
△E
Figure 2- 9 A schematic diagram of electrical field distribution for STR-PD while applied higher external field.
37
Moreover, demanded thickness of conductor layer (intrinsic GaAs) could be thinner due to designed thickness of recombination center, thus higher saturation currier could be expected while operated under this condition. As shown in figure 2-11, higher external electrical field mainly applied in collector layer which located on both side of LTG-GaAs having lots of defects and behavior like heavy doping.
External electrical field is avoided to fall into recombination center, which avert life time increasing effect mentioned above under high external electrical field. Besides, under high power excitation, screen effect induced by accumulated electrical holes in absorption area would weaken electrical field in absorption center. However, the recombination center could trap carriers rapidly thus increase speed performance and maximum saturation current under high power excitation.
2-4 Uni-Traveling-Carrier Photodiode (UTC-PD)
In order to solve the problem of lower quantum efficiency for LTG-GaAs based PD, we propose a novel design which has high speed and quantum efficiency by optimum doping concentration of absorption layer— uni-carrier transportation GaAs/AlGaAs Photodiode (GaAs/AlGaAs based UTC PD). The above mentioned travelling wave type PD is adopted in geometry structure for the novel device. In the other side, its epi-layer structure is shown in figure 2-12 that absorption layer is located in the p-doping area. The structure of UTC-PD includes P-type absorption layer with narrow energy band and N-type light doping with broad energy band as collector layer. Because quasi-neutral P-type absorption layer could relax major carriers (electrical holes) to contact metal, electrons are the only transmission carriers in UTC-PD.
38
Figure 2- 10Schematic epi-layer structure of GaAs/AlGaAs based UTC-PD
The transmission time of electrons determine transmission time of UTC-PD.
Because it’s fast for electrons to pass through conductor layer, we can reduce RC bandwidth limitation without sacrificing thickness of absorption area. Both electrons and electrical holes are not to be sacrificed and thus quantum efficiency could be increased. Moreover, a different space charge screening effect compared with traditional photo detector formed by electrons in intrinsic layer, which provided UTC-PD high output saturation current and high speed performance.
Figure 2- 11 A schematic diagram of UTC-PD while electrical holes relaxed by anode
39
Recently, InGaAs-InP based UTC-PD has been widely used in 1550 nm wavelength which is suitable for optical communication.[20] However, under light excitation with wavelength of 800nm, InGaAs-InP based UTC-PD has a problem that too much unwanted carriers generated in collector layer after InP absorbed light easily.
Such unnecessary electrical pairs left in absorption area induce screening effect influence speed performance of device seriously, which has shown in figure 2-14.
Figure 2- 12A schematic band diagram of UTC-PD while incident light of wavelength 800nm
Figure 2- 13A schematic diagram of UTC-PD design
To solve above problem, we design a collector layer with broad energy band (Eg~1.61eV) which could avoid unnecessary electron pairs generated in collector layer.[31] Moreover, the absorption layer is designed by optimizing doping
40
concentration of absorption layer which shape of energy band become stair step as shown in figure 2-15.
Such design could increase self-induced electrical field and speed up electrons to pass through conductor layer. Besides, we utilize the effect of UTC-PD that bandwidth increasing with optical power to surpass bandwidth performance of traditional PIN PD. The using of GaAs/AlGaAs in absorption layer and transmission layer make our device a high performance sub-THz transmitter at wavelength range of 850nm.
41
2-5 STR-PD Integrated with Slot Antennas
Both low-temperature-grown GaAs (LTG-GaAs) [6][7] and Uni-traevling-carrier photodiode (UTC-PDs)[6] based photonic-transmitters capture many attentions due to their excellent high-speed and high-power performance. However, under high reverse bias voltages, both devices usually suffer from the saturation of output THz power. Such phenomenon can be attributed to the intervalley scattering of photo-generated electrons and life-time increasing effect [9] for the cases of UTC-PDs and LTG-GaAs based photodetectors, respectively. On the other hand, for most reported photonic-transmitters operating in millimeter and sub-millimeter wavelengths regimes, Si lenses are usually required for increasing the antenna radiation efficiency [6,7,33]. However, this integration will not only increase the cost of packaging but decrease some THz radiation due to the coupling loss into dielectric material [10]. In addition, the distance from the center to the edge of the Si-lens is also an obstacle to the optical beam coupled into the side-illumination photodetectors.
Both low-temperature-grown GaAs (LTG-GaAs) [6][7] and Uni-traevling-carrier photodiode (UTC-PDs)[6] based photonic-transmitters capture many attentions due to their excellent high-speed and high-power performance. However, under high reverse bias voltages, both devices usually suffer from the saturation of output THz power. Such phenomenon can be attributed to the intervalley scattering of photo-generated electrons and life-time increasing effect [9] for the cases of UTC-PDs and LTG-GaAs based photodetectors, respectively. On the other hand, for most reported photonic-transmitters operating in millimeter and sub-millimeter wavelengths regimes, Si lenses are usually required for increasing the antenna radiation efficiency [6,7,33]. However, this integration will not only increase the cost of packaging but decrease some THz radiation due to the coupling loss into dielectric material [10]. In addition, the distance from the center to the edge of the Si-lens is also an obstacle to the optical beam coupled into the side-illumination photodetectors.