CHAPTER 2. Rate Equations and laser dynamics
2.3 Relative intensity noise
The measurement of relative intensity noise (RIN) describes the laser’s maximum available amplitude range for signal modulation and serves as a quality indicator of laser devices. RIN can be thought of as a type of inverse carrier-to-noise-ratio measurement. RIN is the ratio of the mean-square optical intensity noise to the square of the average optical power:
Hz
where <∆P2> is the mean-square optical intensity fluctuation (in a 1-Hz bandwidth) at a specified frequency, and P is the average optical power. It is more convenient to define RIN per unit bandwidth because the measurement bandwidth can vary under different experimental conditions. As defined, RIN is measured in dB/Hz.
Relative intensity noise measurements represent the alternative technique used for studies of high-speed dynamics in semiconductor lasers. Like modulation response technique described above this approach allows for determining of dG/dn and K-factor through numerical fitting procedure. However, the signal measured is not response to the external modulation of the bias current but noise spectra of the laser itself. This noise power is associated with the fluctuation of the concentrations in photon and electron systems caused by spontaneous light emission events. Deviation of the electron and photon densities from equilibrium values leads to their damped oscillations with the frequency of electron-phonon resonance and damping factor.
Hence, the measurements of the laser noise power spectra at different bias currents provide enough information to determine such important laser parameters as differential gain and K-factor.
Small signal analysis applied to the rate equations, but taking into account internal noise source and transport of carriers leads to the following expression for the spectral dependence of the RIN:
)
where - δfst - Schawlow-Townes linewidth and * g - differs from g in denominator due to carrier transport through the active region of modern MQW laser .
The main advantage of the RIN technique with respect to modulation response measurements is the elimination of the problem by high frequency modulation experimental equipment. At the same time, low level of the noise for high quality lasers makes it is necessary to use low noise preamplifiers. Also, there is no low
frequency rolloff in the RIN spectra, which is related to the carrier capture and diffusion across SCH time.
References
1. BAHAA E. A. SALEH, MALVIN CARL TEICH, “ Fundamentals of Phtonics,”
John Wiley & Sons, Inc., 1991.
2. Siu Fung Yu, “ Analysis and design of vertical cavity surface emitting lasers,”
John Wiley & Sons, Inc., 2003.
3. Govind P. Agrawal, Niloy K. Dutta, “Semiconductor lasers,” 2nd edition, AT & T, 1993.
4. Marc Xavier Jungo, “ Spatiotemporal VCSEL Model for Advanced Simulations of Optical Links,” Hartung-Gorre Verlag Konstanz, 2003.
CHAPTER 3 Experimental facility and setup
3.1 Inductively coupled plasma reactive ion etching
The ICP etching equipment was a planar ICP-RIE system (SAMCO ICP-RIE 101iPH) as shown in Figure 3-1. The ICP main system is consisted of the source chamber and plasma chamber. The source chamber is constructed with RF generation and matching unit including vacuum pumping system, gas transportation system, and cooling water system.
(1) Plasma source system
The ICP power and bias power source with RF frequency were set at 13.56 MHz.
The output RF power introduce into the tornado coil through impedance match and RF power transmission line. The high density plasma was formed by the tornado coil using the theory of inductively coupled plasma.
The tornado coil was fixed in the source chamber connecting with ground, and the source chamber has the electromagnet field shielding effect. Between the source chamber and plasma chamber, there is the quartz window to be as the separation.
(2) Vacuum pumping system
The vacuum pumping system was constructed with mechanical pump and turbo pump. There is a automatic pressure controller (APC) between the plasma chamber and pump. It could control the pumping rate by tuning the throttle inside the APC.
The throttle and the pressure meter on the plasma chamber assemble a feedback control system.
(3) Gas transportation system
Gas transportation system controls the flow rate of the gas source by the mass flow controller (MFC) and the entering of the gas source is decided by a control valve.
The MFC and control valve can control the flux and time of the gas source. During the experiment, the pressure of the plasma chamber is set by tuning the APC and the MFC which control the flow rate of the gas source.
(4) Cooling water system
During the experiment, some equipment must be continuously cooling and sure to be normal operating to prevent the damage, for example the RF generation and turbo pump. And the source chamber and plasma chamber should not be too hot; they also need to remove the heat by the circulating of the cooling water system.
(5) Wafer transportation system
In the lab, the load lock chamber is set for keeping the high vacuum of the plasma chamber and enhances the convenience of the operation. The wafer transportation system contains the load lock chamber, the gate valve, the transportation arm.
3.2 Ion implantation system
The basic requirement for an ion-implantation system is to deliver a beam of ions of a particular type and energy to the surface of a wafer. Figure 3-2 shows a schematic view of a medium-energy ion implanter. Following the ion path, we begin with the left-hand-side of the system with the high-voltage enclosure containing many of the system components. A gas source feeds a small quantity of source gas such as BF3
into the ion source where a heated filament causes the molecules to break up into charged fragments. This ion plasma contains the desired ion together with many other species from other fragments and contamination. An extraction voltage, around 20 kV, causes the charged ions to move out of the ion source into the analyzer. The pressure in the remainder of the machine is kept below at 10-6 Torr to minimize ion scattering by gas molecules. The magnetic field of the analyzer is chosen such that only ions with the desired charge to mass ratio can travel through without being blocked by the analyzer walls. Surviving ions continue to the acceleration tube, where they are accelerated to the implantation energy as they move from high voltage to ground.
Apertures ensure that the beam is well collimated. The beam is then scanned over the surface of the wafer using electrostatic deflection plates. The wafer is offset slightly from the axis of the acceleration tube so that ions neutralized during their travel will not be deflected onto the wafer. A commercial ion implanter is typical 6m long, 3m wide, and 2m high, consumes 45 kW of power, and can process 200 wafers per hour (dose 1015 ions/cm2).
Three quantities define an ion implantation step: the ion type, energy, and dose.
Given an appropriate source gas, the ion type is determined by magnetic field of the analyzing magnet. In a magnetic field of strength B, ions of charge Q move in a circle of radius R, where
Q V M Q
RB M1 2 1
=
= ν
(3-1) where V is the ion velocity and V is the source extraction voltage. The magnetic field is adjusted so that R corresponds to the physical radius of the magnet for the desired
ion. It is possible for other ions to be accepted if they have a similar value for M1/Q, but since the source provides ions with decrease the beam current. The selected ions are accelerated to the implantation energy by the voltage applied to the acceleration tube.
The total number of ions entering the target per unit area is called the dose. If the current in the ion beam is I, then for a beam swept over an area A, the dose Φ is given by
∫
=
Φ Idt
QA 1
(3-2)
where the integral is over time t. Completing the circuit between target and on source allows the current to be measured. For an accurate current reading, care
must be taken to recapture secondary electrons emitted from the target by incident ions. A Faraday cage around the target, at a small positive bias voltage collects this charge so that it can be included. Wafers frequently patterned surface layers of silicon dioxide, which is a good insulator. Implantation can charge up insulated regions of the surface high enough for dielectric breakdown to occur, which damages the materials.
If the wafer is not well grounded, charging of the whole wafer can distort the ion beam. To avoid this effect, a low-energy election beam can be directed onto the target surface during implantation. The electrons are drawn to charging regions where they neutralize the charge buildup.
Any implantation machine has design limits to its energy range. The minimum implantation energy is usually set by the extraction voltage, which cannot be reduced too far without drastically reducing beam current. Some special machines can operate in a deceleration mode, in which ions are extracted with a normal voltage but then slowed down in a reverse-biased "acceleration" tube resulting in energies as low as 5 KeV. A more common technique uses implants of molecular ions containing the required dopant, for example, BF2+
ions could be implanted at 30 KeV. When the molecule hits the target surface it immediately breaks up into its components, and the energy is divided according to the relative masses. In this case, we would have one 7 keV boron atom and two 11 keV fluorine atoms. The resulting profiles are the same as if 7 keV boron and 11 keV fluorine had been directly implanted, because the binding energy of the molecule is negligible compared to the implantation energy. If channeling is significant, the profiles will be different from that of 7 keV boron alone because the addition of fluorine will increase the lattice damage and reduce channeling. The fluorine distribution is shallower than the boron distribution, so ions can still enter channels when they have traveled past the fluorine peak. Fluorine has only a small effect on annealing and on electrical mobility in the final device.
The maximum implantation energy is set by the design of the high-voltage
equipment. The only way to circumvent this is to implant a multiply-charged ion such as B++. This ion would receive twice the energy of B+ from the same accelerating potential, effectively doubling the energy of the machine. The price paid is a reduced beam current since the number of B++ ions in the source plasma is much smaller.
Fig. 3-2 Schematics diagram of a typical commercial ion-implantation Fig. 3-1 Schematic diagram of inductively coupled plasma reactive ion
Process chamber Matching unit
Matching unit
ICP power 13.56 MHz 1kW
Bias power 13.56 MHz 500W
Phase shifter
Automatic pressure controller LL chamber
Ar
Max 50sccm
ＴＭＰ RP
Ar + e* → Ar+ + 2e- Cl2 + e* → 2Cl + e-
GaN + nCl → GaCl3(s) + NCl3(s)
Cl2 Max 50sccm
3.3 Oxidation process
Fig. 3-3 Schematics of oxidation furnace setup.
Fabrication of selectively oxidized monolithic VCSEL is determined by the layer compositions, and thus the oxidation rates, such that specific oxide layers near the laser cavity have a greater lateral extent to define an oxide aperture. The oxidation selectively to Al composition is exploited to fabricate the buried oxide layers within a VCSEL: With minute changes in Ga concentration, one or more AlGaAs layers are induced to oxidize more rapidly and thus form buried oxide layers for electrical and optical confinement. The selectively oxidation process begins with the deposition of electrical contacts. A silicon nitride mask is deposited and patterned on the wafer top to encapsulate the metal contact and to form an etch mask. Dry etching, such as
reactive ion etching, is used to define mesas or holes to expose the oxidation layers.
Oxidation process was performed in furnace under 400oC~ 450oC, as shown in Figure 3-3. For mesa structures, the oxide aperture is formed by oxide layer extending from the edge into the center of the mesa, as shown in Figure 3-4. For etched hole planar structures, the aperture is formed by the overlap of oxides that extend outward from each hole. The lateral oxidation extent of a layer within the etched mesa or surrounding the etched hole is controlled by the composition of the layer and the oxidation time. Typically, an oxidation temperature of 440 oC produces a oxidation rate of approximately 1 µm/min for the Al0.98Ga0.02As layers. After oxidation the nitride mask is removed to permit laser testing.
Input 100 oC H2O +N2for process
Input N2for purge Pumping line
Valve
Input N2for purge Pumping line
Valve
The oxidation front of an oxide aperture within a VCSEL mesa is shown is Figure 3-5. For a given oxidation time and thus lateral extent of oxidation, variation of the oxide aperture area within a sample can be obtained using differing mesa sizes or
separation between the etched holes. For mesa-etched oxide structures, planarization can be accomplished using polyimide as a backfill or an airbridge technology can be used to allow deposition of metal interconnects. A planar hole oxide structures, but electrical isolation between VCSELs must still be accomplished by some technique such as stacked ion implantation.
Fig. 3-4 SEM image of cross section of oxidized VCSEL.
Fig. 3-5 Optical microscope image of top view of oxidized VCSEL.
3-4 Probe station and spectrum measurement system
Probe station system was essential instrument for basic device characteristics measurement such as I-L (current versus light output), I-V (current versus voltage).
Scheme of probe station system, as illustrated in Figure 3-6, include probe station, current source, and power-meter module. Keithley 238 supplies continuous current for diode laser and receives relative voltage synchronously. The laser was temperature controlled by a precise thermoelectric controller and the output power was measured by Newport power-meter module (model 1835C). An integration sphere was used to pick up whole emitting power from VCSEL to improve the accuracy of power measurement. The integration sphere was calibrated with 850nm, 1300nm, and 1550nm and all the measured values can be traced to NIST standard.
The VCSEL device was placed on a platform of the probe station and was injected bias current with a microprobe. Threshold condition, slope efficiency, turn-on voltage and differential resistance can be obtained from L-I-V information by sweeping bias current. Emission spectrum was measured by optical spectrum analyzer (OSA, Advantest 8381). A multi-mode fiber probe was placed close to the emission aperture to take optical spectra. The OSA had spectrum resolution of 0.1nm which was adequate to measure VCSEL lasing spectra. Scheme of spectrum measurement system was combined with probe station as Figure 3-6.
CCD
Objective Keithley 238 driver
Power supply Power meter
Spectrum analyzer
Computer
Integrating sphere PD
Prober
Fig. 3-6 Probe station measurement instrument setup 3-5 Far-field and near-field
Near-field pattern (NFP) of VCSELs was magnified by high resolution optics
and projected into a B/W CCD then digitized by computer. Beam-view analyzer is useful software we used for the image process of the NFP. We could obtain NFP image under threshold, as spontaneous emission, to define oxide aperture size.
Far-field angle measurement was achieved by scanning the detector cross the laser emission, as shown in Figure 3.7, and recording the intensity with angle simultaneously. A stereo microscope was installed to assistant the wafer-level far-field test.
probe
Scanning photodetector
VCSEL Divergence angle test
machine
probe
Scanning photodetector
VCSEL Divergence angle test
machine
Fig. 3-7 Photograph of far-field measurement setup
3-6 Microwave test system 3.6.1 Setup
The microwave test system was mainly consisted of network analyzer, optical platform and microwave probe, as illustrated with Figure 3-8. Agilent 8720ES network analyzer was a crucial instrument of this microwave measurement.
Transmitter of network analyzer produced -10dBm RF signal. Laser diode drivers (Newport, model 525) provided direct bias current to set the laser above the threshold.
Bias-Tee combined AC and DC signal transmission through the same coaxial cable.
The mixed signal was transmitted through the coplanar microwave probe and was injected into VCSEL, which was bonded on RF submount, as Figure 3-9. The coplanar microwave probe had 700µm pitch (ground (G) to signal (S) tip spacing) and suitable measurement frequency range was up to 40 GHz. Signal probe was higher than ground tips. Fine probing was observed by signal probe skating on contact substrate. Illustration of microwave probe holder in whole was shown in Figure 3-10.
Optical platform contained microscope, beam splitter, objective and fiber coupler etc. We used a long work distance objective (20X, Mitutoyo), fixed in a triple-divide translation stage, to pick up the laser output from VCSEL. Light was separated by
beam splitter and received by CCD and another 10X objective lens(Olympus). One of splitting light was received by a simplified microscope, which was constructed by beam splitter and a CCD to make probing easily. Another light path trough 10X objective coupling into multimode fiber by five-axis fiber aligner (Newport).
The collected light was transmitted into 12 GHz photodetector (Model 1580, NewFocus) and was conversed into electrical signal and fed to network analyzer.
Comparing two channels microwave signal by network analyzer, information of transmission and reflection characteristics could be expressed as vector (magnitude and phase), scalar (magnitude only), or phase-only quantities, that was, S-parameter.
Bias-Tee
VCSEL (Device Under Test) substrate
VCSEL (Device Under Test) substrate
bonding wire
VCSEL (Device Under Test) substrate
Pattern generator
Fig. 3-8 Microwave test system setup
OM vertical view image VCSEL Bonding wire
AlN
1521µm
Au
1050µm 279µm 456µm
Fig. 3-9 Sub-mount substrate design
Photodetector
Laser driver Network analyzer
Optical platform
Microwave probe
Fig. 3-10 Illustration of microwave test system
3.7 Relative Intensity Noise measurement
The experimental setup of RIN is shown in Figure 3-11. The laser was driven by a low noise current source and was temperature controlled by a precise thermoelectric controller. The laser output was collimated and focused by two objective lenses. A beam splitter guided ~ 5% of the laser light into an image-formation lens then the image was recorded by a CCD camera. A multimode fiber was coated with Al thin film on one facet with reflectivity of 10%. The fiber was held by a 5-axises precision fiber positioner and the coated facet was taken as the reflecting mirror. The reflected beam was collimated and re-focused back to the emission aperture of the VCSEL. The tilt of fiber- facet was carefully aligned and formed an external cavity between the top DBR of VCSEL and the fiber-facet. Size of reflected spot can be controlled by the Z-axis of the positioner while the overlapping between the reflected spot and emission aperture was controlled by tilt of the positioner. The external cavity length was about 28 cm. Mirror M1 guide the laser to a fast photodetector (PD, Newfocus 1601) and its output is coupled to a 0.5 GHz oscilloscope (Agilent Infinium) to exam the stability of laser. The output of fiber was connected to a 12 GHz photodetector (NewFocus 1580-A) and its output was coupled
to a RF spectrum analyzer (HP 8563E).
Fig. 3-11 Relative Intensity Noise test system setup
The main problem with the measured noise in the RF spectrum analyzer is that it also represents the shot noise power caused by the quantum nature of the photodetection and the receiver’s total measured thermal noise power. Hence in order
to quantify the shot noise power and the receiver thermal noise power we use, the experimental procedure as described in following.
In order to measure the shot noise power and the receiver thermal noise power,
In order to measure the shot noise power and the receiver thermal noise power,